The glottalic theory is that Proto-Indo-European had ejective stops, , instead of the plain voiced ones, as hypothesized by the usual Proto-Indo-European phonological reconstructions. A forerunner of the theory was proposed by the Danish linguist Holger Pedersen in 1951, but he did not involve glottalized sounds. While early linguists such as André Martinet and Morris Swadesh had seen the potential of substituting glottalic sounds for the supposed plain voiced stops of Proto-Indo-European, the proposal remained speculative until it was fully fleshed out simultaneously but independently in theories in 1973 by Paul Hopper of the United States and by Tamaz V. Gamkrelidze and Vyacheslav Ivanov of the Soviet Union in the journal Phonetica in 1973. The glottalic theory "enjoyed a not insignificant following for a time, but it has been rejected by most Indo-Europeanists." The most recent publication supporting it is Allan R. Bomhard (2008 and 2011) in a discussion of the controversial Nostratic hypothesis, and its most vocal proponents today are historical linguists at the University of Leiden. An earlier supporter, Theo Vennemann, has abandoned the glottalic theory because of incompatibilities between it and his theory of a Semitic substrate and loanwords in Germanic and Celtic languages (Vennemann 2006). However, Martin Kümmel (2012), although rejecting the ejective hypothesis as implausible, argues for a re-interpretation of these stops as implosive, comparable to the Leiden interpretation as pre-glottalized stops. Traditional reconstruction The traditional reconstruction of Proto-Indo-European includes the following stop consonants: is parenthesized because it is at best very rare and perhaps nonexistent. Historically, the inventory was not introduced as an independent proposal, but it arose as a modification of an earlier, typologically more plausible theory. In the original Proto-Indo-European proposal, there was a fourth phonation series, voiceless aspirated , which was assumed to exist on the basis of what is found in Sanskrit, which was then thought to be the most conservative Indo-European language. However, it was later realized that the series was unnecessary and that it was generally the result of a sequence of a tenuis stop () and one of the Proto-Indo-European laryngeal consonants: , , or . The aspirate series was removed, but the breathy voiced consonants remained. Problems There are several problems with the traditional reconstruction. Firstly, the rarity of is odd from a typological point of view. If a single voiced stop is missing from a phoneme inventory (a 'gap'), it would normally be that is