crange string firstExpr lastExpr
Returns a range of characters from string starting
at the character indexed by the expression first-
Expr (zero-based) until the character indexed by
the expression lastExpr.
If the expression firstExpr or lastExpr 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 provided by Extended Tcl.