question: @ Override public Value concatenate ( Value v ) { if ( v instanceof StringVal ) { return ValueFactory . create ( str + " " + v . toString ( ) ) ; } else if ( v instanceof DoubleVal ) { return ValueFactory . create ( str + " " + v . toString ( ) ) ; } else if ( v instanceof NoneVal ) { return this ; } else { log . warning ( "cannot concatenate " + this + " and " + v ) ; return ValueFactory . noneValue ; } }
target:  Returns the concatenation of the two values.