{llena de * el marco de una matriz}
procedure marco (var matriz:mat);
var i,j:integer;
begin
for i:=1 to N do
begin
for j:=1 to N do
begin
if ((i=1) and (j=1)) or ((i=n) and (j=n)) then
matriz[i,j]:= '*';
if ((j=n) and (i<j)) or ((i=j) and (i<j)) then
matriz[i,j]:= '*';
if ((i=n) and (j<i)) or ((j=1) and (j<i)) then
matriz[i,j]:= '*';
end;
end;
end;