/*
   ?- p(light).
yes
   ?- s([push,replace],P).
P = 0.5 ?
yes
   ?- s([push,light],P).
P = 0.5 ?
yes
   ?- s([push,light,replace],P).
P = 0 ?
yes
   ?- s([light,replace],P).
P = 0 ?
yes
   ?- s([light],P).
P = 0.5 ?
yes
   ?- s([replace],P).
P = 0.5 ?
yes
*/

push.
light : 0.5 :- push.
replace :- \+ light.