* bugfix: rpn mode, `=' evaulated wrong stack level (oops!)
* changed "Inv" button to "1/x".
"inv" ie inversion is the more correct operation, because it will apply to all types (eg matrices), whereas 1/x is not always mathematically the same. however, "inv" was being understood to mean the "inv" button of calculators. ie to press "inv" "tan" for arctan etc. this is not the function. nevertheless, now inversions is to be signified by the "1/x" keycap, but insert the "inv(x)" operation into the expression.
* fixed the assignment operator (current ":="), although its not much use in this version.
* bugfix: could store incomplete expressions in memory (eg 1+ .. )
* bugfix: memory recal should often be shown in brackets to avoid ambiguity.
* bugfix: redundant brackets around single numbers confused it.
previously this was intended, eg 10.00000 meant zeros were
genuine zeros. but since the user knows the target precision, there's
no point in spelling this out, also it looks untidy and takes up more
space on the display.
initial test release.