[SP + n] missing

I get an error when trying to assemble any of the following statements:
    SET A, [SP]
    SET A, [--SP]
    SET A, [SP++]
    SET A, [SP + 1]
These are PEEK, PUSH, POP, and PICK n, respectively, which all work correctly.

I was able to work around the issue by using the latter forms, but it seems the earlier forms should be usable as well.
