presto-main/src/main/java/io/prestosql/sql/relational/VariableReferenceExpression.java/VariableReferenceExpression/accept
class VariableReferenceExpression:    @Override
    public <R, C> R accept(RowExpressionVisitor<R, C> visitor, C context)
    {
        return visitor.visitVariableReference(this, context);
    }