Add procedure pair? defined in 6.3.2  Pairs and lists.

procedure:  (pair? obj)

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.2
Use haskeline to support a rich user interface for REPL.

Haskeline provides a rich user interface for line input such as auto-completion
and history. Make REPL use the default setting of haskeline.