

          csubstr string firstExpr lengthExpr
               Returns a range of characters from string starting at
               the character indexed by the expression firstExpr
               (zero-based) for lengthExpr characters.  The special
               keyword end may be specified for lengthExpr to indicate
               that the remainder of the string is to be extracted.
               This command is a short cut for:

                   string range string [expr firstExpr] [expr length-1]
