Cancelations with the same canonical form

[wwitzel]

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Rather than ensuring sub-expressions are identical in order to cancel them (in a sum or product/division), we can do this based upon canonical forms. This has been implemented for Add (in the QPE_finale_witzel branch) but not yet for Mult and Divide. Cancelation theorems need to be changed for this to work flexibly. For example, rather than just having "a - a = 0". The same needs to be done for multiplicative cancelations.