A.java

@JRubyMethod
public IRubyObject delete_prefix(ThreadContext context, IRubyObject arg) {
    RubyString prefix = arg.convertToString();

    if (!this.start_with_p(context, prefix).isTrue()) return this;

    RubyString result = (RubyString) substr19(context.runtime, prefix.strLength(), this.strLength() - prefix.strLength());
    
    return result.isEmpty() ? this : result;
}