Equality: less automatic simplification and fix 'Eq(p, 0)' for p positive

update: no longer on top of #7959 and #7838.
For the record, the crucial bit on line 84 is the simplify() to allows x's on each side to cancel.  See b6a8c7e98494a30493ead0e13cabe985f9e1bffb.  Must remember to think about this, if I'm reworking this on top of #7960.