* cpu/play.cpu (add): Use (ifield enum) for one format element.

	* cpu/play.cpu (add): Use (ifield enum) for one format element.
	* cpu/play.opc: New file.

	Allow arbitrary enums in instruction formats, e.g. (f-op1 OP1_4).
	* insn.scm (-parse-insn-format-ifield-spec): Recognize (ifield enum).
	* doc/rtl.texi (Instructions): Update.