at:

Primitive. Assumes receiver is indexable. Answer the value of an
indexable element in the receiver. Fail if the argument index is not an
Integer or is out of bounds. Essential. See Object documentation
whatIsAPrimitive.

at:put:

Primitive. Assumes receiver is indexable. Store the argument value in
the indexable element of the receiver indicated by index. Fail if the
index is not an Integer or is out of bounds. Or fail if the value is not of
the right type for this kind of collection. Answer the value that was
stored. Essential. See Object documentation whatIsAPrimitive.