Abstract: PCT No. PCT/CH78/00025 Sec. 371 Date June 5, 1979 Sec. 102(e) Date May 22, 1979 Pct Filed Oct. 3, 1977 PCT Pub. No. WO79/00181 PCT Pub. Date Apr. 19, 1979An expander is described having a pair of substantially U-shaped actuating arms. Each of the arms have a pair of branches connected together forming at one end a U member. The U members comprise a first pair of transoms. The arms are pivotally connected together near the transoms. A second pair of transoms connecting the branches of each arm support resilient structure, the resilient structure maintaining a tension between each of the transoms of the second pair with transoms of the first pair, whereby respective arms are biased apart.