Fuzzy logic is logic with truth values that are not necessarily 0 and 1, but somewhere in between. Here's code, mostly by dkf as seen in the Tcl chatroom on 2003-12-05:
proc fuzzyAnd {x y} {expr {$x*$y}}
proc fuzzyNot {x} {expr {1.-$x}}
proc fuzzyNAND {x y} {expr {1.-$x*$y}}
proc fuzzyTrue {} {expr {1.}}
proc fuzzyFalse {} {expr {0.}}
proc fuzzyOr {x y} {fuzzyNot [fuzzyAnd [fuzzyNot $x] [fuzzyNot $y]]}
proc fuzzyXor {x y} {
fuzzyOr [fuzzyAnd $x [fuzzyNot $y]] [fuzzyAnd [fuzzyNot $x] $y]
}
proc fuzzyEq {x y} {fuzzyNot [fuzzyXor $x $y]}