GROUPING ARGUMENTS WITH BRACES
          Curly braces may also be used for grouping arguments.   They
          are  similar  to  quotes except for two differences.  First,
          they nest; this makes them easier  to  use  for  complicated
          arguments  like  nested  Tcl  command  strings.  Second, the
          substitutions described below for commands,  variables,  and
          backslashes do not occur in arguments enclosed in braces, so
          braces can be used to prevent substitutions where  they  are
          undesirable.  If an argument field begins with a left brace,
          then the argument ends at the  matching  right  brace.   Tcl
          will  strip  off  the outermost layer of braces and pass the
          information between the braces to the  command  without  any
          further modification.  For example, in the command

               set a {xyz a {b c d}}
          the set command will receive two arguments: a and xyz a {b c
          d}.

          When braces or quotes are in effect, the matching  brace  or
          quote  need not be on the same line as the starting quote or
          brace; in this case the newline  will  be  included  in  the
          argument  field  along  with  any other characters up to the
          matching brace or quote.   For  example,  the  eval  command
          takes  one argument, which is a command string; eval invokes
          the Tcl interpreter to  execute  the  command  string.   The
          command

               eval {
                    set a 22
                    set b 33
               }

          will assign the value 22 to a and 33 to b.

          If the first character of a command  field  is  not  a  left
          brace,  then neither left nor right braces in the field will
          be  treated  specially   (except   as   part   of   variable
          substitution; see below).