Casting is a form of puppeteering, making a visible vessel act in your stead.

create a blue spell
enter the blue spell
program transform into a fish
leave
create a cat
take the blue spell
cast the blue spell on the cat

Here's a simple discard spell.

This spell can be fired at unwanted vessels, sending them into a vessel discard bin, to be recycled later.

create a recycling bin
create a discard spell
enter the discard spell
program warp to the recycling bin
leave
take the discard spell
cast the discard spell on the unwanted vessel