Method require

chapter17/src/main/java/com/seaofnodes/simple/Parser.java:1203–1203

(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);