A merge rule specifies an ordered operation for combining adjacent token symbols into a larger symbol. Repeatedly applying the most frequent pair builds subwords, from characters toward common words. section 14.2.2
Merging frequent pairs
Starting with character tokens, the frequent pair “o w” becomes “ow”; later, “ow” can combine with “l” to form “low”. The procedure records each pair in application order. section 14.2.2
