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	over dup 0= swap 256 >= or if 2drop 0 ret then
	over addr-pad + set8 1+ ;

\ Add UTF-8 chunk byte to the pad. The 'nn' parameter is the shift count.
: pad-append-UTF8-chunk ( off val nn -- off )
	>> 0x3F and 0x80 or pad-append ;

\ Test whether a code point is invalid when encoding. This rejects the
\ 66 noncharacters, and also the surrogate range; this function does NOT
\ check that the value is in the 0..10FFFF range.
: valid-unicode? ( val -- bool )
	dup 0xFDD0 0xFDEF between? if drop 0 ret then
	dup 0xD800 0xDFFF between? if drop 0 ret then
	0xFFFF and 0xFFFE < ;

\ Encode a code point in UTF-8. Offset is in the pad; it is updated, or
\ set to 0 on error. Leading BOM are ignored.
: encode-UTF8 ( val off -- off )
	\ Skip leading BOM (U+FEFF when off is 1).
	dup2 1 = swap 0xFEFF = and if swap drop ret then

	swap dup { val }
	dup valid-unicode? ifnot 2drop 0 ret then
	choice
		dup 0x80 < uf pad-append enduf
		dup 0x800 < uf
			6 >> 0xC0 or pad-append
			val 0 pad-append-UTF8-chunk
		enduf
		dup 0xFFFF < uf
			12 >> 0xE0 or pad-append
			val 6 pad-append-UTF8-chunk
			val 0 pad-append-UTF8-chunk