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 Th20:
  for f being FinSequence for n st n in dom f & n > 1
    holds n-1 in dom f
proof
  let f be FinSequence;
  consider l being Nat such that
A1: dom f = Seg l by FINSEQ_1:def 2;
  thus thesis by A1,FINSEQ_3:12;
end;
