[ESS] A smart ESS function for replacing "=" used for assignment by "<-"

[Douglas Bates]

I am editing code from another person who uses "=" as the assignment
operator.  For the sake of my sanity I want to change those uses of
"=" to "<-" but, of course, only those uses of "=".  I can use
query-replace but I seem to remember a smart ESS function that would
replace those instances of "=" but not other instances.  I can't find
such a function in the documentation.  Am I hallucinating or is there
such a function?