\llap - Used to create left overlap.
{ \llap #1 }
\llap command creates a box of width zero; the argument is then placed just to the left of this zero-width box (and hence will overlap whatever lies to the left).
a\mathrel{{=}\llap{/}}b
a\mathrel{{=}\llap{/\,}}b
a=\mathrel{\llap{/\,}}b