exp_util.ads (Find_Prim_Op): Document that Program_Error is raised if no primitive operation is found.

2008-07-30  Robert Dewar  <dewar@adacore.com>

	* exp_util.ads (Find_Prim_Op): Document that Program_Error is raised
	if no primitive operation is found.

	* exp_util.adb: (Find_Prim_Op): Add comments for previous change

From-SVN: r138329