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Commands in the Listener are statements in Lil, Decker's scripting language. If this is your first exposure to Lil, you may want to have a look at The Phield Notes  or the Lil reference manual.

Breaking those examples down a bit more,

"%%IMG3..." is a string representation of an image. The image[] function constructs an Image Interface from such a string. In Lil, the colon (:) is an assignment operator. You can read it aloud as "gets" or "becomes". The phrase

i:image["%%IMG3..."]

Constructs an Image Interface and assigns it to a variable named "i". When you define variables like this in the Listener, you can refer to them in subsequent queries, or additional expressions in one query:

Both Images and Canvas widgets have a ".size" attribute giving their dimensions:

Sizes are represented as a list of two numbers- the width and height, respectively, in pixels. In Lil, all arithmetic operators have uniform precedence, applying right to left unless overridden by parentheses. Arithmetic operators also generalize across lists; subtracting them subtracts them element by element. Multiplying a list by a single number "spreads" that number to each element of the list:

Offsetting the top left corner of the pasted image by half of the difference between the size of the image and the canvas will center the image within the canvas.

Instead of specifying a position for drawing our pasted image with (x,y) coordinates, we could specify a position AND a width and height to stretch/crunch the image to fit, like so:

i:image["%%IMG3..."]
thatCanvas.clear[]
thatCanvas.paste[i (0,0),(thatCanvas.size)]

Is that a bit clearer?