ClearHeads is an option of FCLoopIsolate,
FCDiracIsolate and other functions. It takes a list of
heads that will be replaced by Identity in the isolating
function. This is useful for cases when we first apply the isolating
function to an expression, then simplify the isolated expression and
finally want to apply the isolating function again to pull out the
simplified expressions out of the old heads.
Overview, FCLoopIsolate, FCDiracIsolate.