Fix 'add-child' to work even though there's no clauses yet.
Allows 'from' to take multiple statements.
Add some supports of compose-statements for FROM clause that has multiple statements.
Fix scoped-clause of composed-statement.
Fix to surround a subquery in FROM clause with a pair of parenthesis.
Add more examples for 'from' clause at README.
Fix an example of 'fields' clause.