error: detected cyclic import while importing "b1.proto"
  --> a1.proto:4:1
   |
 4 | import "b1.proto";
   | ^^^^^^^^^^^^^^^^^^ imported here
   |
  ::: b1.proto:4:1
   |
 4 | import "c1.proto";
   | ------------------ ...which imports "c1.proto"...
   |
  ::: c1.proto:4:1
   |
 4 | import "d1.proto";
   | ------------------ ...which imports "d1.proto"...
   |
  ::: a1.proto:5:1
   |
 5 | import "b2.proto";
   | ------------------ ...which imports "b2.proto"...
   |
  ::: b2.proto:4:1
   |
 4 | import "c2.proto";
   | ------------------ ...which imports "c2.proto"...
   |
  ::: c2.proto:4:1
   |
 4 | import "d2.proto";
   | ------------------ ...which imports "d2.proto"...
   |
  ::: d2.proto:4:1
   |
 4 | import "b1.proto";
   | ------------------ ...which imports "b1.proto", completing the cycle

encountered 1 error
