What are the elements of unlawful possession of a controlled substance?

"The essential elements of the offense of unlawful possession of a controlled substance are actual or constructive possession in an amount sufficient to be used as a controlled substance with knowledge of its presence and its nature as a controlled substance.... [] Actual or constructive possession is the right to exercise dominion and control over the contraband or the right to exercise dominion and control over the place where it is found. [Citation.] Exclusive possession is not necessary. A defendant does not avoid conviction if his right to exercise dominion and control over the place where the contraband was located is shared with others." ( People v. Rushing (1989) 209 Cal.App.3d 618, 621-622, 257 Cal.Rptr. 286.)

" 'A person aids and abets the commission of a crime when he ... (i) with knowledge of the unlawful purpose of the perpetrator, (ii) and with the intent or purpose of committing, facilitating, or encouraging commission of the crime, (iii) by act or advice, aids, promotes, encourages or instigates the commission of the crime.' [Citation.]" ( People v. Hill (1998) 17 Cal.4th 800, 851, 72 Cal.Rptr.2d 656, 952 P.2d 673.)