Show the rule/equation it will use for the next step

[anka-213]

For example, in this example from the blog post

Prev Next (step 1, next step: apply enumFromTo)

case enumFromTo 1 3 of { 
      []    -> 0
      x:xs' -> length (1 + 0) xs'
}

It would be very neat to also see the definition of enumFromTo and a note of what the values are. Something like this:

Will apply enumFromTo, with n = 1 and m = 3:
enumFromTo = (\n -> \m ->
  if le n m then Cons n (@enumFromTo (add n 1) m)
            else Nil
)

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This should probably be a separate feature request but it would also be very helpful to highlight the parts of the code where the rule will be applied. In particular when the next step is selecting a branch of a case expression. To be extra clear, we could highlight the parameters of the rule with different colours.

[edsko]

#22 add highlighting of the term in the heap that will take a step (but not the exact rule).