ComputerCraft - Write Centered Text

function writeCenter(text, y, mon)
  local w, h = mon.getSize() or term.getSize()
  if mon then
    mon.setCursorPos( math.ceil( w/2 - #text/2 + 1 ), y )
    mon.write( text )
  else
    term.setCursorPos( math.ceil( w/2 - #text/2 ), y )
    term.write( text )
  end
end