/*
   ?- s([heads(C)],P).
C = coin1
P = 0.51 ? ;
C = coin2
P = 0.51 ? ;
no
   ?- s([tails(C)],P).
C = coin1
P = 0.49 ? ;
C = coin2
P = 0.49 ? ;
no

s([tails(coin1)],P).
P = 0.49 ?
*/

heads(Coin): 0.5; tails(Coin) : 0.5:-toss(Coin),fair(Coin).
heads(Coin): 0.6 ; tails(Coin) : 0.4:-toss(Coin),biased(Coin).
fair(Coin):0.9 ; biased(Coin):0.1.
toss(coin1).
toss(coin2).
