expose dispatch to tests
 test parsing
 default hashes for input and output
 test error handling
 add unit parsing
 add argument and result conversion
 getting big, label sections
 retrieve conversions
 debug shows inputs and outputs
 compute product, cancel units
 compute ratios, invert and multiply units
 print units in hover and show
 coerce sums to later unit
 invert factors to apply reverse conversion
 add expression evaluation
 add variables to expressions
 admit dots in label (for fractional numbers)
 separate test traces in log
 find variables by partial name
 parens in printed units