let r be real number ; :: thesis: for M being non empty MetrStruct
for p being Element of M holds Ball p,r = { q where q is Element of M : dist p,q < r }
let M be non empty MetrStruct ; :: thesis: for p being Element of M holds Ball p,r = { q where q is Element of M : dist p,q < r }
let p be Element of M; :: thesis: Ball p,r = { q where q is Element of M : dist p,q < r }
ex M9 being non empty MetrStruct ex p9 being Element of M9 st
( M9 = M & p9 = p & Ball p,r = { q where q is Element of M9 : dist p9,q < r } ) by Def15;
hence Ball p,r = { q where q is Element of M : dist p,q < r } ; :: thesis: verum