▁ad her ence ▁was ▁essential ▁or ▁that ▁a ▁substitute ▁or ▁slight ▁variation ▁would ▁not ▁affect ▁the ▁in vention . ▁A ▁person ▁sk illed ▁in ▁the ▁art ▁is ▁a ▁Canadian ▁legal ▁fiction ▁used ▁in ▁pat ent ▁law ▁that ▁is ▁meant ▁to ▁denote ▁someone ▁of ▁practical ▁skill ▁and ▁knowledge ▁in ▁the ▁subject ▁matter ▁of ▁the ▁pat ent , ▁such ▁as ▁a ▁techn ician . ▁A ▁Person ▁Sk illed ▁in ▁the ▁Art ▁is ▁similar ▁to ▁the ▁American ▁standard ▁of ▁Person ▁Having ▁Ord inary ▁Sk ill ▁in ▁the ▁Art . ▁ ▁A ▁pat ented ▁in vention ▁is ▁thus ▁separated ▁into ▁essential ▁and ▁non - ess ential ▁elements . ▁For ▁an ▁element ▁to ▁be ▁class ified ▁as ▁non - ess ential , ▁it ▁must ▁be ▁true ▁that ▁either : ▁The ▁words , ▁interpreted ▁pur pos ively , ▁make ▁it ▁clear ▁that ▁the ▁element ▁is ▁not ▁essential , ▁or ▁That , ▁as ▁of ▁the ▁date ▁of ▁publication , ▁a ▁person ▁sk illed ▁in ▁the ▁art ▁would ▁have ▁known ▁of ▁a ▁substitute ▁or ▁variation ▁that ▁would ▁not ▁have ▁affected ▁how ▁the ▁in vention ▁works ▁ ▁Any ▁element ▁that ▁is ▁not ▁class ified ▁as ▁non - ess ential ▁is ▁considered ▁essential . ▁It ▁is ▁important ▁to ▁note ▁that ▁if ▁a ▁pat ent ▁is ▁clear ▁that ▁an ▁element ▁is ▁essential , ▁then ▁that ▁element ▁will ▁be ▁essential ▁even ▁if , ▁at ▁the ▁time ▁of ▁publication ▁of ▁the ▁pat ent , ▁an ▁element ▁could ▁be ▁easily ▁subst it uted ▁or ▁varied . ▁ ▁In fr ing ement ▁It ▁is ▁an ▁in fr ing