// Can't just do new Numeric(length) because it always uses
// the constructor for Length!
// Otherwise, must make each class know about Numeric...
// ie, return length.asNumeric(): cleaner
if (length instanceof PercentLength)
return new Numeric((PercentLength)length);
if (length instanceof TableColLength)
return new Numeric((TableColLength)length);
return new Numeric(length);
}