Improve description of SIMPLICITY_ERR_HIDDEN

[uncomputable]

simplicity/C/deserialize.c

Line 46 in 5e79763

* Returns 'SIMPLICITY_ERR_HIDDEN' if the decoded node has illegal HIDDEN children.

It is unclear what illegal children are.

Non-case nodes may not have hidden children, so these children are illegal. However, a case node may not have two hidden children, so these children are also illegal. SIMPLICITY_ERR_HIDDEN is returned in both cases.

[apoelstra]

HIDDEN children are illegal if they don't occur as part of an assertion.

Agreed, this text should be improved.

[roconnor-blockstream]

illegal HIDDEN children is formally defined in

simplicity/C/dag.h

Lines 308 to 314 in 5e79763

	* and for all 'i', 0 <= 'i' < 'len',
	*
	* if dag[dag[i].child[0]].tag == HIDDEN then dag[i].tag == ASSERTR
	*
	* and
	*
	* if dag[dag[i].child[1]].tag == HIDDEN then dag[i].tag == ASSERTL

(technically that defines legal HIDDEN children).

I suppose I can break the various conditions of well-formedness into their own names. But I'm not going to copy the entire formal definition into the error message.

Let me see what I can do.

[uncomputable]

Maybe "The parent of a hidden node is not an assertion." Assertions are defined to be case nodes with one left or right hidden child.