nl_tokens: A number with a unit
pl_tokens: function ( value , unit ) { this . value = parseFloat ( value ) ; if ( isNaN ( this . value ) ) { throw new Error ( 'Dimension is not a number.' ) ; } this . unit = ( unit && unit instanceof Unit ) ? unit : new Unit ( unit ? [ unit ] : undefined ) ; this . setParent ( this . <mask> , this ) ; }