:: deftheorem Def18 defines + QUATERNI:def 18 :
for x being Real
for y being Quaternion
for b₃ being Number holds
( b₃ = x + y iff ex y1, y2, y3, y4 being Real st
( y = [*y1,y2,y3,y4*] & b₃ = [*(x + y1),y2,y3,y4*] ) );