add adding-machine addition appearance bulky buttons calculate calculating calculations calculators clear Clearance computers computing digits handle heavy input large limitations limited machine numbers numerals office office-equipment output predating press pressing problem-solving problems slow subtract subtraction technology bamboo post barbed wire boundary fence off-limit sunset asphalt speed speed limit street clock limit poster time watch border electric grass metal pole post wire away blue clouds keep out security sign signpost signs sky stay warning background car circle red sweden yellow