The implementation of MONGOID-5442 added the $mul operator in a Ruby method named #set_mul.

This name (#set_mul) is incongruent with #inc, #bit, #pop, and other atomic operators for which the Ruby method name exactly matches the native MongoDB operator name.

Unless there is a strong justification as to why the prefix "set_" should be introduced for the $mul operator specifically, we should stick with just #mul to be consistent. There was such a justification for the $min/$max operators whose names conflicted with existing Ruby method names, however no such situation exists for $mul.

PR is raised here: https://github.com/mongodb/mongoid/pull/5527.