src/main/java/com/github/fge/grappa/matchers/join/JoinMatcherBootstrap.java
public JoinMatcherBuilder<V> using(@Nonnull final Object joining)
    {
        return new JoinMatcherBuilder<>(joined, parser, parser.toRule(joining));
    }
public JoinMatcherBuilder<V> using(@Nonnull final Object joining)
    {
        return new JoinMatcherBuilder<>(joined, parser, parser.toRule(joining));
    }
Define the joining rule

@param joining the rule
@return a new {@link JoinMatcherBuilder}

@see BaseParser#toRule(Object)