reserve x,y,y1,y2,z,e,s for set;
reserve alpha,beta,gamma for Ordinal;
reserve n,m,k for Nat;
reserve g,g0,g1,g2,gO,gL,gR,gLL,gLR,gRL,gRR for ConwayGame;

theorem Th47:
  g is zero iff
   (for gL st gL in the_LeftOptions_of g holds gL is fuzzy or gL is negative) &
   (for gR st gR in the_RightOptions_of g holds gR is fuzzy or gR is positive)
by Th45;
