\edef does not work with \protectRobust LaTeX commands are either "naturally robust" - meaning that
they never need \protect, or "self-protected" - meaning that
they have \protect built in to their definition in some
way. Self-protected commands are robust only in a context where the
\protect mechanism is properly handled. The body of an
\edef definition doesn't handle \protect properly, since
\edef is a TeX primitive rather than a LaTeX command.
This problem is resolved by a LaTeX internal command
\protected@edef, which does the job of \edef while keeping the
\protect mechanism working. There's a corresponding
\protected@xdef which does the job of \xdef.
Of course, these commands need to be tended carefully, since they're internal: see '@' in control sequence names.
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