Oh, seems useful. I'm generally quite confused about the new commands' syntax..
Thanks for the help! Now I get what the replace in /setblock means. But then why is autofill suggesting that I put a hash '#' here:
/fill 137 67 -147 137 67 -147 minecraft:sand replace #minecraft:grass_block [doesn't work]
/fill 137 67 -147 137 67 -147 minecraft:sand replace minecraft:grass_block [works]
in 18w21a it the grass and mycelium don't spread anymore, however, it doesn't suffocate either...