Sentence:  The argument against this last approach is comparable to that which rejects stories about hoop snakes , about snakes that break themselves into many pieces and join up again , or even of ghosts that chase people out of graveyards ; the mere <WSD>piling</WSD> up of testimony does not prove , to the scientific mind , the existence of hoop snakes , joint snakes , or ghosts . 
Meaning of the word marked with <WSD> tags:  arrange into piles or stacks