Compounds of the following formula:in which R1, R2, A1, A2, X, Y, m, n, p, x and y are as defined herein. The spring steel comprises a tempered martensite and 2. The concrete double-tee includes a deck, a first leg and a second leg. Suitable medical devices include stents. The valve lid is disposed on top of the hollow sleeve portion so that the purging head can move from the first position to the second position inside the fixed sleeve body when being propped up by an upward force.