... 'x{A:B}1y:x{A:B}2y', which expands to'xA1y:xB1y:xA2y:xB2:y'.
I guess the last line should read:
'xA1y:xB1y:xA2y:xB2y'.
Section 4.4 could also mention "none" asa possible TEX_HUSH value.