Not clear about the generic modifiers here: ```inl...

# codereview

[Mark]

Not clear about the generic modifiers here:

inline fun <reified K, V> Map<*, V>.filterKeyIsInstance(): Map<K, V> {
    return filterKeyIsInstanceTo(mutableMapOf())
}

inline fun <reified K, V, M : MutableMap<in K, in V>> Map<*, V>.filterKeyIsInstanceTo(destination: M): M {
    entries.forEach { (key, value) ->
        if (key is K) {
            destination[key] = value
        }
    }
    return destination
}

// should this by `out K` or just `K`?
inline fun <K, reified V> Map<out K, *>.filterValueIsInstance(): Map<K, V> {
    return filterValueIsInstanceTo(mutableMapOf())
}

inline fun <K, reified V, M : MutableMap<in K, in V>> Map<K, *>.filterValueIsInstanceTo(destination: M): M {
    entries.forEach { (key, value) ->
        if (value is V) {
            destination[key] = value
        }
    }
    return destination
}

inline fun <reified K, reified V> Map<*, *>.filterIsInstance(): Map<K, V> {
    return filterIsInstanceTo(mutableMapOf())
}

inline fun <reified K, reified V, M : MutableMap<in K, in V>> Map<*, *>.filterIsInstanceTo(destination: M): M {
    entries.forEach { (key, value) ->
        if (key is K && value is V) {
            destination[key] = value
        }
    }
    return destination
}