let i, j be Nat; for G being Go-board st i <= len G & j <= width G holds
cell (G,i,j) is convex
let G be Go-board; ( i <= len G & j <= width G implies cell (G,i,j) is convex )
assume
( i <= len G & j <= width G )
; cell (G,i,j) is convex
then
( v_strip (G,i) is convex & h_strip (G,j) is convex )
by Th18, Th19;
hence
cell (G,i,j) is convex
by GOBOARD9:6; verum