The REXX Tutorial 

  Note: some of the information in this file is system-specific, though
  most of it pertains to every implementation of Rexx.
# Lines containing implementation-specific information are flagged with
# letters in column 1. The letter "I" denotes information for REXX/imc.
# I have also started adding information about OS/2 Rexx, which will be
# flagged with "O" in column 1. I hope it is not too confusing to see some
# sections written twice, once for each system.  The file is designed so
# that if you run it through "egrep '^(x| |$)'|cut -c3-" (where x is the
# desired letter) then it should still make sense and cut out the flag
# letters.  Doing this will select one of the following lines indicating
# which system was selected; the characters to the left of this paragraph
# will make sure it gets deleted when that happens.
I This file describes REXX/imc.
O This file describes OS/2 Classic Rexx (which is also pretty much
O compatible with the OS/2 Object Rexx interpreter).
  
I More advanced information can be found in rexx.summary (bare details
I of each command and builtin function, with a list of differences from
I standard Rexx) and rexx.ref (technical details of every aspect of this
I Rexx implementation).
O More information is available in the OS/2 help system.  For example,
O typing "help rexx signal" will give the syntax of the Rexx "signal"
O instruction.