Witches and ghosts: Mechanics

Incant: Your witches can help cast this spell. Each witch you tap while casting this spell pays for or one mana of that witch's color. Used on 0 cards:

In this set, I want witches to be able to help cast enchantments. So this is basically a convoke, except that it will only allow witches to tap to help pay the cost. I don't want to use convoke as it is written because I want the witches to also summon ghosts and familiar tokens, and I don't like the flavor of those tokens being able to help cast enchantments.

In card text or details pages, write the mechanic's code name (playtest name) between square brackets, like "[Crittercast]", "[Bushido 1]" or "[Delay 4 {2}{R}{R}]". It will be expanded to the mechanic's name plus reminder text, such as "Suspend 4 – . (Rather than cast this spell from your hand,...)"
To expand a mechanic's name but not its reminder text, include parentheses at the end of the square brackets: "[Crittercast()]", "[Delay 9 {R}()]".