[fexl]

Postscript: You might well ask why not just use "=" only, and assume that all definitions are potentially self-referencing?

That's a non-starter because I find that redefinition is the more common intent:

    \x=4
    \x=(* x 5)
    \x=(+ y x)

In those cases I don't want x defined in terms of itself, but rather in terms of the previous definition of x.

That is why I would insist on a special token such as "==" to express the intention of self-reference.