let i, j be Nat; :: thesis: for D being non empty set
for f being FinSequence of D st i in dom f & j in dom f holds
not mid (f,i,j) is empty
let D be non empty set ; :: thesis: for f being FinSequence of D st i in dom f & j in dom f holds
not mid (f,i,j) is empty
let f be FinSequence of D; :: thesis: ( i in dom f & j in dom f implies not mid (f,i,j) is empty )
assume ( i in dom f & j in dom f ) ; :: thesis: not mid (f,i,j) is empty
then len (mid (f,i,j)) >= 1 by Th5;
hence not mid (f,i,j) is empty by FINSEQ_3:25, RELAT_1:38; :: thesis: verum