  Despite considerable research on document spanners, little is known about the
expressive power of generalized core spanners. In this paper, we use
Ehrenfeucht-Fra\"iss\'e games to obtain general inexpressibility lemmas for the
logic FC (a finite-model variant of the theory of concatenation). Applying
these lemmas give inexpressibility results for FC that we lift to generalized
core spanners. In particular, we give several relations that cannot be selected
by generalized core spanners, thus demonstrating the effectiveness of the
inexpressibility lemmas. As an immediate consequence, we also gain new insights
into the expressive power of core spanners.
