let G be Go-board; :: thesis: for i1, j1, i2, j2 being Nat st 1 <= i1 & i1 <= len G & 1 <= j1 & j1 + 1 <= width G & 1 <= i2 & i2 + 1 <= len G & 1 <= j2 & j2 <= width G & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) meets LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2))) & not ( j1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))} ) holds
( j1 + 1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))} )

let i1, j1, i2, j2 be Nat; :: thesis: ( 1 <= i1 & i1 <= len G & 1 <= j1 & j1 + 1 <= width G & 1 <= i2 & i2 + 1 <= len G & 1 <= j2 & j2 <= width G & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) meets LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2))) & not ( j1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))} ) implies ( j1 + 1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))} ) )
assume that
A1: ( 1 <= i1 & i1 <= len G ) and
A2: ( 1 <= j1 & j1 + 1 <= width G & 1 <= i2 & i2 + 1 <= len G ) and
A3: ( 1 <= j2 & j2 <= width G & LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1)))) meets LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2))) ) ; :: thesis: ( ( j1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))} ) or ( j1 + 1 = j2 & (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))} ) )
per cases ( j1 = j2 or j1 + 1 = j2 ) by A1, A2, A3, Th21;
case A4: j1 = j2 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))}
now :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))}
per cases ( i1 = i2 or i1 = i2 + 1 ) by A1, A2, A3, Th21;
suppose i1 = i2 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))}
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))} by A2, A4, Th17; :: thesis: verum
end;
suppose i1 = i2 + 1 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))}
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))} by A2, A4, Th18; :: thesis: verum
end;
end;
end;
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,j1))} ; :: thesis: verum
end;
case A5: j1 + 1 = j2 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))}
now :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))}
per cases ( i1 = i2 or i1 = i2 + 1 ) by A1, A2, A3, Th21;
suppose i1 = i2 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))}
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))} by A2, A5, Th15; :: thesis: verum
end;
suppose i1 = i2 + 1 ; :: thesis: (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))}
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))} by A2, A5, Th16; :: thesis: verum
end;
end;
end;
hence (LSeg ((G * (i1,j1)),(G * (i1,(j1 + 1))))) /\ (LSeg ((G * (i2,j2)),(G * ((i2 + 1),j2)))) = {(G * (i1,(j1 + 1)))} ; :: thesis: verum
end;
end;