Problem: User defined functions can't be a closure.
Solution: Add the "closure" argument. Allow using :unlet on a bound
variable. (Yasuhiro Matsumoto, Ken Takata)
Problem: filter() and map() either require a string or defining a function.
Solution: Support lambda, a short way to define a function that evaluates an
expression. (Yasuhiro Matsumoto, Ken Takata)