diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0a959cc..c526250 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,8 +2,6 @@ cmake_minimum_required(VERSION 3.19)
 
 project(test_add_target)
 
-include(add_target.cmake)
-
 macro(get_cmake_test)
     include(cmake_test/cmake_test OPTIONAL RESULT_VARIABLE cmake_test_found)
     if(NOT cmake_test_found)
@@ -30,4 +28,5 @@ get_cmake_test()
 
 include(cmake_test/cmake_test)
 
-ct_add_dir("test")
+enable_testing()
+ct_add_dir("test" USE_REL_PATH_NAMES)
diff --git a/fetch_source_files.cmake b/fetch_source_files.cmake
deleted file mode 100644
index c22ceff..0000000
--- a/fetch_source_files.cmake
+++ /dev/null
@@ -1,62 +0,0 @@
-include(add_target.cmake)
-include(unit_test_for_cmake/unit-test.cmake)
-
-function(test_sources_alone)
-  fetch_source_files(sources test.cpp)
-  expect_streq("test.cpp" "${sources}")
-
-  fetch_source_files(sources "test.cpp;foo.txt")
-  expect_streq("test.cpp;foo.txt" "${sources}")
-
-  fetch_source_files(sources "test.cpp" "foo.txt")
-  expect_streq("test.cpp;foo.txt" "${sources}")
-
-  fetch_source_files(sources)
-  expect_empty("${sources}")
-
-  fetch_source_files(out_sources "test.cpp")
-  expect_streq("test.cpp" "${out_sources}")
-
-  fetch_source_files(out_sources "test.cpp;foo.txt")
-  expect_streq("test.cpp;foo.txt" "${out_sources}")
-endfunction()
-
-function(test_groups_alone)
-  fetch_source_files(sources "///")
-  expect_empty("${sources}")
-
-  fetch_source_files(sources "//")
-  expect_empty("${sources}")
-
-  fetch_source_files(sources "//" "a//" "/b//" "a/b/c//" "/a/b//")
-  expect_empty("${sources}")
-
-  fetch_source_files(sources "//;a//;/b//;a/b/c//;/a/b//")
-  expect_empty("${sources}")
-endfunction()
-
-function(test_options)
-  fetch_source_files(sources "test.cpp:header" "test.txt:file")
-  expect_streq("test.cpp;test.txt" "${sources}")
-
-  fetch_source_files(sources "test.cpp:header;test.txt:file")
-  expect_streq("test.cpp;test.txt" "${sources}")
-
-  fetch_source_files(
-    sources "test.cpp:header;test.txt:file//") # last one interpreted as group
-  expect_streq("test.cpp" "${sources}")
-endfunction()
-
-function(test_all_together)
-  fetch_source_files(sources "test.h" "b//" "test.cpp:header" "/a:c//"
-                     "test.txt")
-  expect_streq("test.h;test.cpp;test.txt" "${sources}")
-
-  fetch_source_files(sources "test.h;b:c//;test.cpp:header;/a//;test.txt")
-  expect_streq("test.h;test.cpp;test.txt" "${sources}")
-endfunction()
-
-test_sources_alone()
-test_groups_alone()
-test_options()
-test_all_together()
diff --git a/fetch_source_groups.cmake b/fetch_source_groups.cmake
deleted file mode 100644
index 5439fc0..0000000
--- a/fetch_source_groups.cmake
+++ /dev/null
@@ -1,64 +0,0 @@
-include(add_target.cmake)
-include(unit_test_for_cmake/unit-test.cmake)
-
-function(test_no_groups)
-  fetch_source_groups(sources groups "")
-  expect_empty("${groups}")
-  expect_empty("${sources}")
-
-  fetch_source_groups(sources groups "test.cpp")
-  expect_empty("${groups}")
-  expect_empty("${sources}")
-
-  fetch_source_groups(out_sources out_groups "")
-  expect_empty("${out_sources}")
-  expect_empty("${out_groups}")
-
-  fetch_source_groups(out_sources out_groups "test.cpp")
-  expect_empty("${out_sources}")
-  expect_empty("${out_groups}")
-endfunction()
-
-function(test_no_sources)
-  fetch_source_groups(sources groups "toto//")
-  expect_empty("${groups}")
-  expect_empty("${sources}")
-
-  fetch_source_groups(sources groups "/toto//;tata//")
-  expect_empty("${groups}")
-  expect_empty("${sources}")
-
-  fetch_source_groups(out_sources out_groups "toto//")
-  expect_empty("${out_sources}")
-  expect_empty("${out_groups}")
-
-  fetch_source_groups(out_sources out_groups "/toto//;tata//")
-  expect_empty("${out_sources}")
-  expect_empty("${out_groups}")
-endfunction()
-
-function(test_main)
-  fetch_source_groups(
-    sources groups
-    "test.cpp;toto//;test.h;detail/impl.h;tata//;help.txt;/foo/bar//;readme.md")
-  expect_streq("${groups}" "toto;toto;toto/tata;foo/bar")
-  expect_streq("${sources}" "test.h;detail/impl.h;help.txt;readme.md")
-
-  fetch_source_groups(
-    out_sources out_groups
-    "test.cpp;toto//;test.h;detail/impl.h;tata//;help.txt;/foo/bar//;readme.md")
-  expect_streq("${out_groups}" "toto;toto;toto/tata;foo/bar")
-  expect_streq("${out_sources}" "test.h;detail/impl.h;help.txt;readme.md")
-
-  fetch_source_groups(sources groups "")
-  expect_empty("${groups}")
-  expect_empty("${sources}")
-
-  fetch_source_groups(out_sources out_groups "")
-  expect_empty("${out_groups}")
-  expect_empty("${out_sources}")
-endfunction()
-
-test_no_groups()
-test_no_sources()
-test_main()
diff --git a/add_target.cmake b/io1_add_target.cmake
similarity index 82%
rename from add_target.cmake
rename to io1_add_target.cmake
index 27c7f17..18ffe43 100644
--- a/add_target.cmake
+++ b/io1_add_target.cmake
@@ -163,7 +163,7 @@ function(add_target target_name)
 endfunction()
 
 set(source_group_regex "^(.*)//$")
-set(source_file_properties_regex "^(.*):(.*)$")
+set(source_file_properties_regex "^(.+):?(.*)$")
 
 function(apply_source_groups)
   fetch_source_groups(sources groups ${ARGN})
@@ -227,9 +227,11 @@ function(fetch_source_files out_sources)
       PARENT_SCOPE)
 endfunction()
 
+
 function(apply_source_files_properties)
   foreach(str IN LISTS ARGN)
-    if("${str}" MATCHES "${source_group_regex}")
+    is_source_group("${str}" out)
+    if(out)
       continue()
     endif()
 
@@ -245,29 +247,63 @@ function(apply_source_files_properties)
   endforeach()
 endfunction()
 
+# adds src to target, applying options
+function(io1_add_source_file target src)
+	io1_parse_file_options("${src}" file options)
+
+	get_target_property(type ${target} TYPE)
+	if (${type} STREQUAL "INTERFACE_LIBRARY")
+		target_sources(${target} INTERFACE ${file})
+	else()
+		target_sources(${target} PRIVATE ${file})
+	endif()
+
+	cmake_parse_arguments(io1 "cpp;header" "" "" ${options})
+	if(io1_cpp)
+		set_source_files_properties("${file}" PROPERTIES LANGUAGE CXX)
+	endif()
+	if(io1_header)
+		set_source_files_properties("${file}" PROPERTIES HEADER_FILE_ONLY ON)
+	endif()
+endfunction()
+
+# any string that ends with // is a source group
+function(io1_is_source_group str res)
+	if("${str}" MATCHES "^(.*)//$")
+		set(${res} "TRUE" PARENT_SCOPE)
+	else()
+		set(${res} "FALSE" PARENT_SCOPE)
+	endif()
+endfunction()
+
 # expects "file.c[:opt1,opt2,opt3,...]" and parse it into "file.c" and the list
 # "opt1;opt2;opt3"
-function(parse_file_options str out_file out_options)
-  if("${str}" MATCHES "${source_file_properties_regex}")
-    set(${out_file}
-        "${CMAKE_MATCH_1}"
-        PARENT_SCOPE)
-    if("${CMAKE_MATCH_1}" STREQUAL "")
-      set(${out_options}
-          ""
-          PARENT_SCOPE)
-    else()
-      string(REPLACE "," ";" temp_out_options "${CMAKE_MATCH_2}")
-      set(${out_options}
-          "${temp_out_options}"
-          PARENT_SCOPE)
-    endif()
-  else()
-    set(${out_file}
-        "${str}"
-        PARENT_SCOPE)
-    set(${out_options}
-        ""
-        PARENT_SCOPE)
-  endif()
+function(io1_parse_file_options str out_file out_options)
+	if("${str}" MATCHES "^(.*):(.*)$")
+		if ("${CMAKE_MATCH_1}" STREQUAL "")
+			message(FATAL_ERROR "${CMAKE_CURRENT_FUNCTION}: ignored orphaned source option '${CMAKE_MATCH_2}'.")
+		else()
+			set(${out_file}
+				"${CMAKE_MATCH_1}"
+				PARENT_SCOPE)
+
+			if("${CMAKE_MATCH_2}" STREQUAL "")
+			  unset(${out_options}
+				  PARENT_SCOPE)
+			else()
+			  string(REPLACE "," ";" temp_out_options "${CMAKE_MATCH_2}")
+			  set(${out_options}
+				  "${temp_out_options}"
+				  PARENT_SCOPE)
+			endif()
+		endif()
+	else()
+		set(${out_file}
+			"${str}"
+			PARENT_SCOPE
+		)
+		unset(${out_options}
+			PARENT_SCOPE
+		)
+	endif()
 endfunction()
diff --git a/test/add_source_file.cmake b/test/add_source_file.cmake
new file mode 100644
index 0000000..0d068f5
--- /dev/null
+++ b/test/add_source_file.cmake
@@ -0,0 +1,28 @@
+include(${CMAKE_CURRENT_LIST_DIR}/../io1_add_target.cmake)
+file(WRITE "${CMAKE_CURRENT_SOURCE_DIR}/foo.cpp" "int main() { return 0; }")
+
+ct_add_test(NAME io1.add_source_file.normal)
+function(${CMAKETEST_TEST})
+	add_library(foo STATIC)
+	set_target_properties(foo PROPERTIES LINKER_LANGUAGE CXX)
+
+	io1_add_source_file(foo "foo.cpp")
+	get_source_file_property(prop "foo.cpp" LANGUAGE)
+	ct_assert_equal(prop "NOTFOUND")
+	
+	get_source_file_property(prop "foo.cpp" HEADER_FILE_ONLY)
+	ct_assert_equal(prop "NOTFOUND")
+endfunction()
+
+ct_add_test(NAME io1.add_source_file.header)
+function(${CMAKETEST_TEST})
+	add_library(foo_header STATIC)
+	set_target_properties(foo_header PROPERTIES LINKER_LANGUAGE CXX)
+
+	io1_add_source_file(foo_header "foo.cpp:header")
+	get_source_file_property(prop "foo.cpp" LANGUAGE)
+	#ct_assert_equal(prop "NOTFOUND")
+	
+	get_source_file_property(prop "foo.cpp" HEADER_FILE_ONLY)
+	ct_assert_equal(prop "ON")
+endfunction()
diff --git a/test/is_source_group.cmake b/test/is_source_group.cmake
new file mode 100644
index 0000000..331f6ae
--- /dev/null
+++ b/test/is_source_group.cmake
@@ -0,0 +1,56 @@
+include(${CMAKE_CURRENT_LIST_DIR}/../io1_add_target.cmake)
+
+ct_add_test(NAME io1.is_source_group.normal)
+function(${CMAKETEST_TEST})
+	set(out "foo")
+	io1_is_source_group("test.cpp" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("test" out)
+	ct_assert_false(out)
+	
+	set(out "foo")
+	io1_is_source_group("" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("/" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("/foo" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("/foo/" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("/foo/bar" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("/foo/bar/" out)
+	ct_assert_false(out)
+
+	set(out "foo")
+	io1_is_source_group("//" out)
+	ct_assert_true(out)
+
+	set(out "foo")
+	io1_is_source_group("/foo//" out)
+	ct_assert_true(out)
+
+	set(out "foo")
+	io1_is_source_group("foo//" out)
+	ct_assert_true(out)
+
+	set(out "foo")
+	io1_is_source_group("bar/foo//" out)
+	ct_assert_true(out)
+
+	set(out "foo")
+	io1_is_source_group("/bar/foo//" out)
+	ct_assert_true(out)
+endfunction()
diff --git a/test/parse_file_options.cmake b/test/parse_file_options.cmake
index 8ff13a1..0724aa9 100644
--- a/test/parse_file_options.cmake
+++ b/test/parse_file_options.cmake
@@ -1,66 +1,53 @@
-include(cmake_test/cmake_test)
+include(${CMAKE_CURRENT_LIST_DIR}/../io1_add_target.cmake)
 
-ct_add_test(NAME parse_file_options.main)
+# variable names out_file and out_options are also local names
+# of parse_file_options, keep them in sync
+ct_add_test(NAME io1.parse_file_options.normal)
 function(${CMAKETEST_TEST})
-  parse_file_options("test.cpp" file options)
-  ct_assert_equal(file "test.cpp")
-  ct_assert_not_defined(options)
+	set(out_file "foo")
+	set(out_options "bar")
+	io1_parse_file_options("test.cpp" out_file out_options)
 
-  parse_file_options("test.cpp:toto" file options)
-  ct_assert_equal(file "test.cpp")
-  ct_assert_equal(options, "toto")
-
-  parse_file_options("test.cpp:toto,tata" file options)
-  ct_assert_equal(file "test.cpp")
-  ct_assert_equal(options, "toto;tata")
-
-  parse_file_options("test.cpp" file options)
-  ct_assert_equal(file "test.cpp")
-  ct_assert_not_defined(options)
-
-  parse_file_options("" file options)
-  ct_assert_not_defined(file)
-  ct_assert_not_defined(options)
-
-  parse_file_options(":" file options)
-  ct_assert_not_defined(file)
-  ct_assert_not_defined(options)
+	ct_assert_equal(out_file "test.cpp")
+	ct_assert_not_defined(out_options)
+endfunction()
 
-  parse_file_options("test.cpp:" file options)
-  ct_assert_equal(file "test.cpp")
-  ct_assert_not_defined(options)
+ct_add_test(NAME io1.parse_file_options.with_one_option)
+function(${CMAKETEST_TEST})
+	set(out_file "foo")
+	set(out_options "bar")
+	io1_parse_file_options("test.cpp:toto" out_file out_options)
 
-  parse_file_options(":toto" file options)
-  ct_assert_not_defined(file)
-  ct_assert_not_defined(options)
+	ct_assert_equal(out_file "test.cpp")
+	ct_assert_equal(out_options "toto")
 endfunction()
 
-
-# Using same variable names as in function definition because some
-# implementations may break in this case.
-ct_add_test(NAME parse_file_options.same_variable_names)
+ct_add_test(NAME io1.parse_file_options.with_two_options)
 function(${CMAKETEST_TEST})
-  parse_file_options("test.cpp" out_file out_options)
-  ct_assert_equal(out_file "test.cpp")
-  ct_assert_not_defined(out_options)
+	set(out_file "foo")
+	set(out_options "bar")
+	io1_parse_file_options("test.cpp:toto,tata" out_file out_options)
 
-  parse_file_options("test.cpp:toto" out_file out_options)
-  ct_assert_equal(out_file "test.cpp")
-  ct_assert_equal(out_options "toto")
+	ct_assert_equal(out_file "test.cpp")
+	ct_assert_equal(out_options "toto;tata")
+endfunction()
 
-  parse_file_options("test.cpp" out_file out_options)
-  ct_assert_equal(out_file "test.cpp")
-  ct_assert_not_defined(out_options "toto")
+ct_add_test(NAME io1.parse_file_options.with_no_option)
+function(${CMAKETEST_TEST})
+	set(out_file "foo")
+	set(out_options "bar")
+	io1_parse_file_options("test.cpp:" out_file out_options)
 
-  parse_file_options(":" out_file out_options)
-  ct_assert_not_defined(out_file)
-  ct_assert_not_defined(out_options)
+	ct_assert_equal(out_file "test.cpp")
+	ct_assert_not_defined(out_options)
+endfunction()
 
-  parse_file_options("test.cpp:" out_file out_options)
-  ct_assert_equal(out_file "test.cpp")
-  ct_assert_not_defined(out_options)
+ct_add_test(NAME io1.parse_file_options.orphaned_option EXPECTFAIL)
+function(${CMAKETEST_TEST})
+	io1_parse_file_options(":toto" out_file out_options)
+endfunction()
 
-  parse_file_options(":toto" out_file out_options)
-  ct_assert_not_defined(out_file)
-  ct_assert_not_defined(out_options)
+ct_add_test(NAME io1.parse_file_options.orphaned_no_option EXPECTFAIL)
+function(${CMAKETEST_TEST})
+	io1_parse_file_options(":" out_file out_options)
 endfunction()