API temporary template variable

[joseph-hellerstein]

tvar(var_name, base, add=additions, remove=removals)

Does a union of additions (a list of strings or other template variables) and a subtraction
of removals (a list of strings or other template variables). var_name is in the namespace
only for this substitution.

[joseph-hellerstein]

tvar(var_name, base=variables, add=additions, remove=removals)

[joseph-hellerstein]

tvar(var_name, base=[], additions=[], removals=[])

[joseph-hellerstein]

Invocation of tvar:

1. Adds a definition to the API
2. returns a value that is the first value
   Current logic must be adapted so that after each variable expression must determine if other variables should be included?

[joseph-hellerstein]

Or, maybe also provide implicit definitions {x,y,z}