Update the documentation for erf_representation to document what it
really does. `true` is not a valid value; it must be one of `erf`,
`erfc`, or `erfi`, which causes the error function to be converted to
the given function.
The expected values were `(nil $erf $erfc $erfi)`, but we need to use
the noun forms `(nil %erf %erfc $erfi) because that's what the
assignment value would be.
`defmvar` now recognizes the `:setting-list` option. This was only
used in `$macroexpansion` (in mmacro.lisp) as `setting-list`. So just
rename it to `:setting-list` there.
Also added error checks so that we can't use `:setting-list`,
`:setting-predicate`, or `:properties` with an 'assign property. Only
one of these is allowed.
Add a new test with programmode = false for solve where solve can find
some roots but not all. This is test that the removal of the special
var `equations` was done correctly.