<Stmt> ::= 'if' '(' <Expr> ')' '{' <Stmt>* '}' ['else' '{' <Stmt>* '}']
| 'while' '(' <Expr> ')' '{' <Stmt>* '}'
| <id> <FieldSels> '=' <Expr> ';'
<Stmt> ::= 'if' '(' <Expr> ')' '{' <Stmt>* '}' ['else' '{' <Stmt>* '}']
| 'while' '(' <Expr> ')' '{' <Stmt>* '}'
| <id> <FieldSels> '=' <Expr> ';'