presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitWhenClause
class AstBuilder:    @Override
    public Node visitWhenClause(SqlBaseParser.WhenClauseContext context)
    {
        return new WhenClause(getLocation(context), (Expression) visit(context.condition), (Expression) visit(context.result));
    }