Define a lock in Mozart

locks.ozf

declare
C = {NewCell 0}
L = {NewLock}

thread I in
   lock L then I in
      I = @C
      C := I + 1
      {Show 'Thread I'}
      {Show C}
   end
end

thread J in
   lock L then I in
      J = @C
      C := J + 1
      {Show 'Thread J'}
      {Show C}
   end
end