|139||139|| * We use !SubstFam and !SubstVar to substitute wanted equalities '''only''' if their right-hand side contains a flexible type variables (which for variable equalities means that we apply !SubstVar only to flexible variable equalities). '''TODO:''' This is not sufficient while we are inferring a type signature as SPJ's example shows: `|- a ~ [x], a ~ [Int]`. Here we want to infer `x := Int` before yielding `a ~ [Int]` as an irred. So, we need to use !SubstVar and !SubstFam also if the rhs of a wanted contains a flexible variable. This unfortunately makes termination more complicated. However, SPJ also observed that we really only need to substitute variables in left-hand sides (not in right-hand sides) as far as enabling other rewrites goes. However, there are trick problems left as the following two examples show `|- a~c, a~b, c~a` and `|- b ~ c, b ~ a, a ~ b, c ~ a`.