csubstr string firstExpr lengthExpr
Returns a range of characters from string starting
at the character indexed by the expression first-
Expr (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 provided by Extended Tcl.