rearranging, regrouping works, factoring 1st attempt
simplify bug fixed
cancel minimally working (but with 2 avoidable bugs)
Merge remote-tracking branch 'origin/rearrange-regroup-factor'
"Between" hypothesis for presentation

The linear pair theorem now requires that the pivot be between the other
two collinear points. A new theorem has been provided to allow this
betweenness to be proven.
Substitution dialog shows LaTeX

The width of the preview and the dropdown are fixed, so long LaTeX will
get cut off. Provisionally, however, it works okay. If necessary, the
table layout can be widened to 3 or 4 columns.