ccmd

CCMD  (C Command Interface)

CCMD is a user interface package, written at Columbia, that was included in
  MM to make it easier type commands.  The two main features are command
  completion and command help.  

Command Completion:  You can type most MM commands with only one or two 
  letters.  CCMD picks the only, or most likely, completion and the command
  is done as if you typed it in full.  For example, at the MM> prompt, "r"
  is completed as "read", while at the Read> prompt, "r" is completed as
  "reply".  To see the completion on screen, press the TAB key: type "r"
  and then the TAB key.  The feature also applies to message-sequences, so
  "headers all" can be typed as just "h a", and default message-sequences
  can be previewed with TAB (for example, type the "h", SPACE and TAB keys).

Command Help:  At any point (except when typing your message text), the
  question-mark ? key brings up a list of possible completions.  For
  example, "?" alone shows a list of all commands; "he?" shows a list of
  commands starting with "he"; and "headers ?" shows a list of all forms
  of message-sequences.