:: deftheorem defines is_derivable_from LANG1:def 5 :
for G being non empty DTConstrStr
for n, m being String of G holds
( m is_derivable_from n iff ex p being FinSequence st
( len p >= 1 & p . 1 = n & p . (len p) = m & ( for i being Element of NAT st i >= 1 & i < len p holds
ex a, b being String of G st
( p . i = a & p . (i + 1) = b & a ==> b ) ) ) );