STACK

$A|Ax|K\ I\ X\ --$
$S\ I\ I\ --$
$D\ X_{key}\ X_{any}\ --$

DESCRIPTION

Red book operator replaces a single element of the value of the first operand.
If the first operand is an array, stack or a string, put treats the second
operand as an index and stores the third operand at the position identified
by the index, counting from 0. I must be in the range 0 to n - 1, where n is
the length of the array, stack or string. If the first operand is a
dictionary, put uses the second operand as a key and the third operand
as a value, and stores this key-value pair into dict. If key is already
present as a key in dict, put simply replaces its value by $X_{any}$ .
Otherwise, put creates a new entry for key and associates $X_{any}$ with it.

ORIGIN

Core code registered in systemdict.