make whitespace more flexible in "to show", etc.

[jwaldmann]

(again, Wadler's law)

some identifiers consist of more that one token. Sometimes they are separated by exactly one space, sometimes by whitespace (e.g., more spaces). (I will mark space by "_" in the following)

• "proof__by_induction" is fine
• "proof_by__induction" is not
• "To__show" is not
• "(by_def__plus)" is not

I think that in all cases, whitespace should be accepted.