BEGIN,
    A 0xff LDri, (HL) A LDrr, ( default mask )
    L1 BSET ( loop2 ) @GET, @PUT,
    0x20 CPi, JRZ, L4 FWR ( escapechar )
	( not an escape char, just apply the mask and write )
    (HL) ANDr, (HL) A LDrr,
    HL INCd,
JR, AGAIN,
L4 FSET ( escapechar, adjust by setting (hl) to 0x7f )
7 (HL) RES, JR, L1 BWR ( loop2 )
L2 FSET ( maybeerror, was it an error? )
A ORr, JRZ, L1 BWR ( loop2, not an error )
L3 FSET ( error )
C A LDrr, ( error code from @GET/@PUT )
A 0x1a LDri, ( @ERROR ) 0x28 RST, RET,