Here's a question for the community. I was recently told by a referee, very succinctly, that "natural languages are sparse." I believe he/she must have meant that natural languages are each members of the SPARSE complexity class of languages, defined here:
While this seems reasonable at a glance, I have been at a loss to find any publication that proves, states, or otherwise mentions this putative fact. Does anybody know how to establish whether natural languages are really sparse?