COMMAND SUBSTITUTION WITH BRACKETS
          If an open bracket occurs in a  field  of  a  command,  then
          command  substitution  occurs (except for fields enclosed in
          braces).  All of the text up to the matching  close  bracket
          is  treated as a Tcl command and executed immediately.  Then
          the result of that command is substituted for the  bracketed
          text.  For example, consider the command

               set a [set b]

          When the set command has only a single argument, it  is  the
          name  of  a  variable  and  set returns the contents of that
          variable.  In this case, if variable b has  the  value  foo,
          then the command above is equivalent to the command

               set a foo
          Brackets can be used in more complex ways.  For example,  if
          the  variable b has the value foo and the variable c has the
          value gorp, then the command

               set a xyz[set b].[set c]

          is equivalent to the command

               set a xyzfoo.gorp

          A bracketed command may contain multiple commands  separated
          by  newlines  or  semi-colons in the usual fashion.  In this
          case the value of the last command is used for substitution.
          For example, the command

               set a x[set b 22
               expr $b+2]x

          is equivalent to the command

               set a x24x

          If a field is enclosed in braces then the brackets  and  the
          characters  between them are not interpreted specially; they
          are passed through to the argument verbatim.