PROCEDURE matsub(a,b:matrix; VAR c:matrix);
VAR i,j : integer;
BEGIN
   c.m := a.m; c.n :=a.n;
   FOR i:=1 TO c.m DO FOR j := 1 TO c.n DO
   c.a[i,j] := a.a[i,j]-b.a[i,j]
END;