@User, the syntax looks quite complicated, however: • using tcleval(..) allows you to decide if the attribute needs to be evaluated further. Since one of the primary goals of xschem is speed you can avoid unneeded processing. for example tcleval(10) is really pointless. • the [expr ... ] is just the tcl syntax for evaluating an expression ( [expr 10 + sin(3)] ). with these 2 items in mind you understand the syntax 🙂