Mr Cooper argues that inter-country variations in price and expenditure structure constitute an insurmountable obstacle to meaningful measurement of differences in the purchasing power of currencies.

ECONOMIST: Letters | The

But the US appears to be growing as an obstacle, with campaigners acknowledging privately that the balance of power in Congress is likely to become even less favourable to carbon-cutting legislation after the next round of elections in 2012.

BBC: Modest hopes for climate summit, as gas levels rise

The regulator will also possess the power to set maximum prices for interconnections, currently seen as being a severe obstacle to the emergence of rival firms in the fixed-line and wireless market.

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Tradition may be an obstacle now, but never underestimate what smart programmers and a lot of cheap processing power can do.

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Libya's interim prime minister has been branded "an obstacle to the revolution" by some of the armed groups that are furiously jockeying for power in post-Gaddafi Tripoli.

BBC: Libya's interim PM feels the heat

Power lines are extremely hostile places for data signals, with high levels of electronic noise and an obstacle course of transformers, surge arrestors and splices that create a huge impedance to the free flow of high-frequency signals.

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With the necessary computing power within reach, and adequate broadband to send that much information over long distances, the final obstacle to developing true holographic video systems is finding a projector or display with fine enough resolution to produce a hologram.

FORBES: Adventures in the Third Dimension