reading a top statement
```ebnf
top_statement ::=
namespace | function | class
| interface | trait
| use_statements | const_list
| statement
```
function() {
    switch (this.token) {
      case this.tok.T_FUNCTION:
        return this.read_function(false, false);
      // optional flags
      case this.tok.T_ABSTRACT:
      case this.tok.T_FINAL:
      case this.tok.T_CLASS:
        return this.read_class();
      case this.tok.T_INTERFACE:
        return this.read_interface();
      case this.tok.T_TRAIT:
        return this.read_trait();
      case this.tok.T_USE:
        return this.read_use_statement();
      case this.tok.T_CONST: {
        const result = this.node("constantstatement");
        const items = this.next().read_const_list();
        this.expectEndOfStatement();
        return result(null, items);
      }
      case this.tok.T_NAMESPACE:
        return this.read_namespace();
      case this.tok.T_HALT_COMPILER: {
        const result = this.node("halt");
        if (this.next().expect("(")) this.next();
        if (this.expect(")")) this.next();
        this.expect(";");
        this.lexer.done = true;
        return result(this.lexer._input.substring(this.lexer.offset));
      }
      default:
        return this.read_statement();
    }
  }