In trying to deftype a structure, I tried to include IDrop, however, the compiler doesn’t recognize this. So I tried things at a repl, and discovered that if I type almost any protocol from cljs.core then it prints a value of #object[Function]. However, this is not the case for IDrop:

cljs.user=> INext
#object[Function]
cljs.user=> ISwap
#object[Function]
cljs.user=> IVolatile
#object[Function]
cljs.user=> IIterable
#object[Function]
cljs.user=> IDrop
WARNING: Use of undeclared Var cljs.user/IDrop at line 1 <cljs repl>
nil