An introduction of CAS
a:=1/2 --- Assignment of variable
Delete[a] --- Deletion of variable
x+y=1 --- Assignment of equation
LeftSide[x+y=1] -> x+y
RightSide[x+y=1] -> 1
# --- Copies the previous output.
#2 --- Copies the output of row 2.
$ --- Inserts a reference to the previous output.
$2 --- nserts a reference to the output of row 2.