public static <K, V> MultiMapBuilder<K, V, Collection<V>> keys(Supplier<? extends Map<K, Collection<V>>> supplier) {
		return new MultiMapBuilder<>(supplier);
	}
Creates new multi-map builder with user-specific keys.

@param supplier
		Map supplier.
@param <K>
		Key type.
@param <V>
		Value type.

@return New builder.
recaf-core/src/main/java/software/coley/recaf/util/MultiMapBuilder.java