new combinator format
Rewrite with new combinator names.
Added example to show time (was 24.564s)
added display of duration (only when longer than a second)
Reorganization of file handling.

We don't keep track of last line anymore, and the last expression gets
re-substituted with _ after parse.

Implemented \a(expr) which results in the a-eliminate of expr.
Apply un-substitution in reverse dependency order.

This doesn't work satisfactory yet. For example, in the expansion of V,
there shows up a T, despite V does not depend on T. And so V is not
recognized anymore. 
At the end of the day 'unsubst' depends on the shape of the tree for
combinators that do not depend on each other.
Read with continuation lines.

A continuation line will be read if the previous line has missing clsing
parentheses.
implement ')' completion
Flexible primitives