Reason for that change: interpret correctly both
  (1) A by B -> C and (2) A -> B by C
  Case (2) is common, and we expect by to take precendence,
  so that A is the context for the proof of B.
  Case (1) is rarer, and is troublesome if by take precendence
  cause (A by B) -> C is almost certainly not what the user
  intended to write. Putting precendence of by/so at the same level as
  the arrow fix this