This one’s noteworthy because, although it’s C code, it’s been “macro-ized” to look like ALGOL. For example, this function from blok.c.

VOID	free(ap)
	BLKPTR		ap;
{
	REG BLKPTR	p;

	IF (p=ap) ANDF p<bloktop
	THEN	Lcheat((--p)->word) &= ~BUSY;
	FI
}