A clause concerning `unused`

[ruv]

forth2012-test-suite/src/coreexttest.fth

Lines 25 to 26 in 9773f84

	\ UNUSED tests revised as UNUSED UNUSED = may return FALSE when an
	\ implementation has the data stack sharing unused dataspace.

(added in the commit 2d0224a)

I don't think unused unused = may return false in a standard Forth system.

If placing a value on the data stack may affect unused, then that is a side effect that must be specified in the standard if it is allowed.

This is because:

• for a standard-compliant program: everything which is not explicitly forbidden by the standard is allowed;
• for a standard-compliant system: everything which can be detected by a standard-compliant program and is not explicitly allowed by the standard is forbidden;

Thus, a standard Forth system shall pass the following test case:

t{ here unused  2dup erase  here unused  d=  -> true }t