UCG repl help

The UCG repl allows you to enter ucg statements interactively. You can import
files create named bindings and see the result of UCG expressions.

The repl understands some non-ucg command as well. Any command that starts with
a '#' character is treated as a ucg repl command and is not parsed as a ucg
statement. Repl commands are invalid inside a ucg statement and will result
in a ucg parse error.

Currently supported commands are:

* #help
  - prints out this help text

* #del <name>
  - deletes a previously created named binding