       csubstr string firstExpr lengthExpr
              Returns  a range of characters from string starting at the char-
              acter indexed  by  the  expression  firstExpr  (zero-based)  for
              lengthExpr characters.

              If the expression firstExpr or lengthExpr starts with the string
              end, then end is replaced with the index of the  last  character
              in  the  string.  If the expression starts with len, then len is
              replaced with the length of the string.

              This command is UTF-aware.
