Support for multiplication in constraint-solving

There is limited support for handling multiplication in constraint-solving.
Heres is an example:

http://www.ats-lang.org/SERVER/MYCODE/Patsoptaas_serve.php?mycode_url=https://raw.githubusercontent.com/githwxi/ATS-Postiats/master/doc/EXAMPLE/ARITH/tally-of-powers.dats

It proves the following identities:

2*(1+2+…+n) = n(n+1)

6*(1^2 + 2^2 + … + n^2) = n*(n+1)(2n+1)

66*(1^10 + 2^10 + … + n^10) = some-polynomial-of-degree-11

In general, only equalities involving multiplication can be handled
automatically.
Inequalities cannot be handled.

Using MUL to handle multiplication manually in proof construction is very
labor-intensive,
and it should be avoided whenever possible.