Add equalized-list type, parsed by [= a,b,c...] syntax

We have a parser and a printer, "not implemented" functions
for everything recursion-related (TODO: test for problems
when the user tries to do this anyway).

Need to write increment/decrement functions which will affect
the ordering of get_priv_data_const
sequence recursion can now use equalized gap-sets

but it appears not to work.
Get equalized list to actually work ;)