Prevent evaluation : quote hold '

A quoted subexpression (either with ' or with the quote or hold) command will not be evaluated.
Remark a:=quote(a) (or a:=hold(a)) is equivalent to purge(a) (for the sake of Maple compatibility). It returns the value of this variable (or the hypothesis done on this variable).
Input :

a:=2;quote(2+3*a)

or

a:=2;'2+3*a'

Output :

(2,2+3.a)