:: deftheorem Def12 defines Rea QUATERNI:def 12 :
for z being Quaternion
for b₂ being Number holds
( ( z in COMPLEX implies ( b₂ = Rea z iff ex z9 being Complex st
( z = z9 & b₂ = Re z9 ) ) ) & ( not z in COMPLEX implies ( b₂ = Rea z iff ex f being Function of 4,REAL st
( z = f & b₂ = f . 0 ) ) ) );