def key(value)
      value = Erlang.from(value)
      each { |entry| return entry[0] if value == entry[1] }
      return nil
    end
def key(value)
      value = Erlang.from(value)
      each { |entry| return entry[0] if value == entry[1] }
      return nil
    end
Searches through the `Map`, comparing `value` with each value (using `#==`).
 When a matching value is found, return its associated key object.
 Return `nil` if no match is found.

 @example
   Erlang::Map["A" => 1, "B" => 2, "C" => 3].key(2)  # => "B"

 @param value [Object] The value to search for (using #==)
 @return [Object]