Method require

chapter13/src/main/java/com/seaofnodes/simple/Parser.java:685–685

(String syntax)

    // Require an exact match
    private Parser require(String syntax) { require(null, syntax); return this; }
    private <N> N require(N n, String syntax) {
        if (match(syntax)) return n;
        throw errorSyntax(syntax);