math.functions: add logit function and a test.

sigmoid is known as expit in scipy

basis/math/functions/functions-tests.factor

@@ -268,6 +268,7 @@ CONSTANT: log10-factorial-1000 0x1.40f3593ed6f8ep11
 { t } [ 3 15 roots [ 15 ^ 3 .01 ~ ] all? ] unit-test
 
 { .5 } [ 0 sigmoid ] unit-test
+{ t } [ 0 [ sigmoid logit ] keep .000001 ~ ] unit-test
 
 { 1 } [ 12 signum ] unit-test
 { -1 } [ -5.0 signum ] unit-test

basis/math/functions/functions.factor

@@ -434,8 +434,12 @@ M: float round-to-odd [ (round-to-odd?) ] (float-round) ;
     [ recip 2pi * 0 swap complex boa e^ ]
     [ <iota> [ ^ * ] 2with map ] tri ;
 
+! expit
 : sigmoid ( x -- y ) neg e^ 1 + recip ; inline
 
+: logit ( x -- y ) [ ] [ 1 swap - ] bi /f log ; inline
+
+
 GENERIC: signum ( x -- y )
 
 M: real signum sgn ;