Mu expressions can miss type checking errors

[nickbattle]

The following spec does not report any TC errors in 2.5.2:

types
	ReceiptLine :: field : nat;

functions
	receiptLines: () -> ReceiptLine
	receiptLines() ==
		let rlines = if true then [ mk_ReceiptLine(1) ] else [] in
			mu(rlines(1), field |-> wibble);

However, "wibble" is undefined. The reason for this is that the conditional creation of "rlines" means that its type is "seq of ReceiptLine" or "seq of ?", and so its elements are ReceiptLine or ?. The presence of the unknown "?" is causing the mu operator to skimp on the type checking, missing the undefined variable - which should be an error, even if we don't know anything about the record type.

[nickbattle]

Now fixed in ncb/development.