question: public < T extends JCTree > void printExprs ( List < T > trees , String sep ) throws IOException { if ( trees . nonEmpty ( ) ) { printExpr ( trees . head ) ; for ( List < T > l = trees . tail ; l . nonEmpty ( ) ; l = l . tail ) { print ( sep ) ; if ( sep . endsWith ( "\n" ) ) { align ( ) ; } printExpr ( l . head ) ; } } }
target:  Derived visitor method: print list of expression trees, separated by given string.