Define the electromeric effect and describe its types.

(N/A) Definition: The electromeric effect is a temporary effect observed in organic compounds containing multiple bonds (double or triple bonds) in the presence of an attacking reagent. It is defined as the complete transfer of a shared pair of $\pi$-electrons to one of the atoms joined by a multiple bond at the demand of an attacking reagent.
Characteristics:
$1$. It is a temporary effect and is annulled as soon as the attacking reagent is removed.
$2$. It is represented by $E$ and the electron shifting is shown by a curved arrow $(\curvearrowleft)$.
Types of Electromeric Effect:
$(a)$ Positive Electromeric Effect $(+E)$: In this effect,the $\pi$-electrons of the multiple bond are transferred to the atom to which the attacking reagent gets attached. Example: Addition of $H^+$ to an alkene.
$(b)$ Negative Electromeric Effect $(-E)$: In this effect,the $\pi$-electrons of the multiple bond are transferred to the atom to which the attacking reagent does $NOT$ get attached. Example: Addition of $CN^-$ to a carbonyl group.