$*$prefix :sets the prefix to a null string so that EXEC have no special indicator. If EXEC recognizes a statement as a directive, the statement is interpreted as an EXEC directive; otherwise, the statement is taken as a system command.
The sequence $*$PREfix is always recognized as a PREfix directive, even if the prefix for directives is not "$*$". This makes it possible to write "portable" EXEC files that can be invoked no matter what the prefix currently is. The first statement in such an EXEC file can be
$*$PREfix :stringsetting the prefix to whatever is used in the rest of the file.
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