By Allan Little
BBC News, Johannesburg
There is a ghost at the table around which the four principal negotiators have been sitting these last three days, trying to resolve Zimbabwe's political crisis.
The talks are haunted by the spirit of the late Joshua Nkomo, whose fate stands as a warning to anyone trying to strike a deal with President Robert Mugabe.
Joshua Nkomo was, broadly, Mr Mugabe's contemporary, and a Zimbabwean liberation leader of impeccable credentials.
In 1980, at independence, he emerged as an alternative leader to Mr Mugabe.
His support base was in Matabeleland in the south and west of the country.
Mr Mugabe fought him for five years.
He destroyed him in two ways. First he sent into Matabeleland the ruthless, North Korea-trained Fifth brigade.
Thousands of Mr Nkomo's supporters were murdered and their bodies dumped in mass graves in a two-year operation known as Gukurahundi.
Then - and this was a master stroke - Mr Mugabe reached an agreement with Mr Nkomo: a power-sharing agreement.
Mr Nkomo was brought into the government as vice-president.
Officially, the two political parties merged to form Zanu-PF, but in reality Mr Mugabe's party swallowed Mr Nkomo's Zapu party whole.
Mr Nkomo was neutralised, destroyed.
Mr Mugabe used what, on the face of it, was sold to the world as a power-sharing agreement to consolidate his own one-party state.
It entrenched his dictatorship for 20 years.
If Mr Nkomo - who died in 1999 - could speak from the grave, would he warn the opposition Movement of Democratic Change (MDC) leader Morgan Tsvangirai not to walk into the same trap?
Mr Mugabe and Mr Tsvangirai have agreed on the need to share power.
Mr Mugabe stays as president, Mr Tsvangirai becomes prime minister.
Mr Tsvangirai holds stronger cards than Mr Nkomo did
But they are deadlocked on how much and what kind of power Mr Mugabe should retain.
Mr Mugabe has in mind what you might call the Nkomo solution: he retains control of the military and security services that he has used so successfully to terrorise his way to successive election victories.
In other words he retains the coercive instruments of real executive power.
Mr Tsvangirai gets the economy to sort out.
Mr Tsvangirai is not weak enough to have to accept this poisoned chalice.
For one thing the European Union and the United States have both made it clear that they would not help fund a recovery package under a deal like this.
Mr Mugabe makes hay with this, accusing his rival of being the candidate of Western interests, of resurgent British imperialism.
This plays well in much of Africa, but it no longer plays well in Zimbabwe, where there is now real economic privation.
On the contrary, the evidence is that there is immense pressure on the MDC from below, from the millions of ordinary Zimbabweans who have risked much and endured more.
If they are afraid of anything now it is that Mr Tsvangirai will be tempted to settle.
Many would see such a deal as an unforgivable betrayal.
At the negotiating table it has been three against one - with Thabo Mbeki of South Africa and Arthur Mutambara, who leads a minority faction of the opposition, joining forces with Mr Mugabe to put pressure on Mr Tsvangirai to accept the Zanu-PF power-sharing plan.
Brave as he is, constancy is not one of Mr Tsvangirai's virtues.
The talks have hung on whether he would bend to this pressure.
There is much dark talk in MDC circles of intolerable bullying.
Thabo Mbeki has been mediating talks in Zimbabwe
But Mr Tsvangirai has not caved. He has shown more backbone than the other three had hoped.
What he wants is the transfer of real executive power from the president's office to that of the prime minister.
Mr Mugabe would stay on as head of state in a largely ceremonial role.
The odds are stacked against that. The hardliners who run the military and security services are implacably against it.
Mr Mugabe is negotiating for them as well as for his party.
But Mr Tsvangirai has two strong cards: the first is that he holds the key to an internationally funded recovery programme, which cannot happen without him; and time is on his side.
In South Africa, Thabo Mbeki has less than a year left in office. His likely successor, Jacob Zuma, has been much more critical of Mr Mugabe, and his party, the African National Congress, has openly accused Mr Mugabe of bringing the liberation tradition into disrepute.
It is in Mr Mugabe's interests to strike a deal before Mr Zuma takes over.
The parallels are not exact - this is not 1987.
Joshua Nkomo did not, then, hold the cards that Morgan Tsvangirai holds now.
Robert Mugabe is finding that it is no longer so easy to swallow the opposition whole and go on governing, unchallenged.