MSIG function extracts a signature macro from a signed macro. The function can also be used with 3
STRING parameters in which case is simply consumes those parameters and emits nothing. This latter case is meant for signature macros so such macros are valid macros which do nothing. See
MSIGN for the format of the signature macros.
If the input macro does not contain a signature, the result of applying
MSIG will be an empty macro.