Consume.string t ~len reads len characters (all, by default) from t into a new string and advances the lower bound of the window accordingly.

Fill.bin_prot X.bin_write_t t x writes x to t in bin-prot form, advancing past the bytes written.

Poke.bin_prot X.bin_write_t t x writes x to the beginning of t in binary form without advancing. You can use X.bin_size_t to tell how long it was. X.bin_write_t is only allowed to write that portion of the buffer you have access to.