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-u: Undefine FWEB macro (FTANGLE)

-uA undefines the FWEB macro A previously defined on the command line (or in .fweb) via -m.

CAUTION: This option can also undefine built-in functions such as $IF. Don't do that, since built-ins can use other built-ins behind the scenes; undefining one can cause very strange behavior.