The British heavy bands of the 70's were open listeners, and influenced by the U.S. West Coast scene plus blues, country, rockabilly, etc.
However, the Stones, and Cream had no problem giving credit.
Zep have had no problem enforcing laws against rappers.
It also is a matter of a progression/riff being a trigger point for a song's composition or a if song has been wholesaledly ripped off and renamed. Zep did both.
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