[[Replace this comment with an explanation of the return value of '''NaN''' that occasionally occurs within Tcl]] [RS] 2007-02-23: ''NaN'' is "Not a Number", a sort-of valid floating point value from Tcl 8.5. Here's experiments: (bin) 13 % set y [expr sqrt(-1)] -NaN (bin) 14 % expr {$y == $y} ;# the only value that is not equal to itself :^) 0 (bin) 15 % expr {$y eq $y} 1 ---- [Category Glossary]