Reserve proxy names of SatML (#836)

The SAT solver SatML produces proxy terms while building the
CNF form of an expression. These terms have top symbols `Sy.Name`
whose the string is prefixed by `PROXY_`.
I replace the prefix by `.PROXY_` as names starting by a dot are
reserved in the SMT-LIB standard.

src/lib/structures/satml_types.ml

@@ -925,7 +925,7 @@ module Flat_Formula : FLAT_FORMULA = struct
     if is_neg then a.Atom.neg,l else a,l
 
   let mk_new_proxy n =
-    let hs = Hs.make ("PROXY__" ^ (string_of_int n)) in
+    let hs = Hs.make (".PROXY__" ^ (string_of_int n)) in
     (* TODO: we should use a smart constructor here. *)
     let sy = Symbols.Name (hs, Symbols.Other, false) in
     E.mk_term sy [] Ty.Tbool
@@ -995,7 +995,7 @@ module Proxy_formula = struct
     if is_neg then a.Atom.neg,l else a,l
 
   let mk_new_proxy n =
-    let sy = Symbols.name @@ "PROXY__" ^ (string_of_int n) in
+    let sy = Symbols.name @@ ".PROXY__" ^ (string_of_int n) in
     E.mk_term sy [] Ty.Tbool
 
   let rec mk_cnf hcons f ((proxies, inv_proxies, new_vars, cnf) as accu) =