[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