If the Imam of Yemen failed to inspire one of Edward Lear's famous limericks, it was only because Lear never heard of him. To this day little is known about this Moslem kingdom, the size of Nebraska, at the southern tip of the Arabian peninsula. That is the way Yemen's despotic ruler, the Imam Saif el Islam Ahmed, wants it. He bars foreigners and does everything he can to keep out of print. But last week there was print without stint: there had been a revolt against the Imam of Yemen. Tough Iraq-trained Colonel Ahmed Thalaya, mindful of army coups in nearby Egypt and Syria, persuaded a bunch of soldiers to surround the royal palace of Al Urdhi at Taiz, a fortified stronghold where the Imam lives with his harem, the royal treasure, an arsenal of modern weapons, and a 150-man guard. From the palace window came a shout: "What is it you want? Tell me what you want," cried the sexagenarian Imam who, in his fringed turbans and silken robes, bears a striking resemblance to Charles Laughton playing Henry VIII. When the soldiers called back that it was time he got off the throne, he dodged back, reappeared with a Tommy gun. But they told him the game was up, the country and the Council of Elders were with them. Suddenly, the old man agreed to abdicate, but demanded that his son Badr succeed him. No, replied Colonel Thalaya, he had another candidate: the King's halfbrother, the Emir Saif el Islam Abdullah, the 48-year-old Foreign Minister, who once represented Yemen at the U.N. While Abdullah set about forming a new government, the old Imam retired within his palace, broke open the treasury coffers and secretly began buying off the besieging soldiers. After five days, the number of besiegers being reduced from 600 to 40, the Imam suddenly burst out of the palace gates flourishing a long scimitar. Before the sentries could get over their shock, he had slashed two of them dead, scrambled back into the palace. Exchanging the sword for a submachine gun, he led his 150 guards onto the roof of the palace and began a direct attack on the rebels. At the end of 28 hours, with 23 rebels and one palace guard dead, Colonel Ahmed Thalaya gave up. Abdullah, guarded by heavily armed slaves, taken for a ride in a jeep in the direction of the rock dungeon of Hajja, was later reported executed. News leaking from Yemen told of the old Imam leading the defeated colonel into the square in front of the palace and crying to the crowd: "Look at this man. I personally sent him to be educated in Iraq. I made him chief of the army. I trusted him. I even let him use my airplane. And now look how he has repaid me! I leave it to you. If you say 'forgive,' I will let him go. If you say otherwise . . ." The mob howled for blood; the colonel's hands were bound, and he was forced to kneel in the dust. As the executioner raised his sword, following an old custom, he gave the kneeling man a passing jab in the shoulder, making him jerk forward so that his neck was stretched out tautly for the downcoming stroke. A minute later the mob fell upon the decapitated body and tore it to pieces. The Imam of Yemen was even.