A command is evaluated in two steps:

 (a) The Tcl interpreter breaks the command into words and performs substitutions as described below.  (Note: These substitutions are performed in the same way for all commands.)

 (b) The first word resulting from (a) is used to locate a command procedure to carry out the command, then all of the words of the command are passed to the command procedure. (Note: The command procedure of (b) is free to interpret each of its words in any way it likes, such as an integer, variable name, list, or Tcl script. Different commands interpret their words differently.)
