Got subscript-functions to work.
Made 'mathDefinition' as freeVarStack-polymorphic (ImpredicativeTypes) as 'mathFuncDefinition' already was. This isn't really fine yet, requires too much explicit signature in the monadic bindings.
Added test case that fails due to floating point issues (prettyFloatApprox uses a hackish and non-total show-parse-process approach.
Fixed buggy prettyFloatApprox. Now without show & re-parse hackery; works but not quite optimally yet; sometimes even fails to recognise exact rationals.
Improved output of prettyFloatApprox. Now produces actual fractions if that's readily possible; in principle nice, though it's not exactly purpose and only works inconsistently (basically for halves and fifths, arising from the decimal expansion used).