Optional vs. mandatory terminators in context-free grammar definition

Question

In a book chapter about compilers, there's the following grammar definition and example code.

...
statement: whileStatement
           | ifStatement
           | ... // Other statement possibilities
           | '{' statementSequence '}'
whileStatement: 'while' '(' expression ')' statement
ifStatement: ... // Definition of "if"
statementSequence: '' // empty sequence (null)
                   | statement ';' statementSequence
expression: ... // Definition of "expression"
...             // More definitions follow

 

while (expression) {
 statement;
 statement;
 while (expression) {
  while(expression)
     statement;
  statement;
 }
}

How is the code's inner-most while loop valid without { }? It looks to me that the statement definition requires them. Is this a mistake in the book or am I misunderstanding the syntax?

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[Edit] My apologies for any ambiguity. Everything typed above is verbatim from the book. The omissions were not my doing.

Answer 1

Your while statement says that after the ) comes a statement. Your grammar doesn't fully specify statement, but it doesn't require braces. Braces are only needed for a statement sequence.