injectors in wasm_eosio_injection.hpp

Question

What are injectors defined in wasm_eosio_injection.hpp? What do they do? The following is a snippet. An illustration on how one injector is used will be very helpful.

   struct pre_op_injectors : wasm_ops::op_types<pass_injector> {
      using call_t            = wasm_ops::call                    <call_depth_check>;
      using call_indirect_t   = wasm_ops::call_indirect           <call_depth_check>;
      // float binops 
      using f32_add_t         = wasm_ops::f32_add                 <f32_binop_injector<wasm_ops::f32_add_code>>;