The present invention provides compounds of formula and pharmaceutically acceptable salts thereof,wherein L, X, Ra, Rb, R1, R2 and R3 are as defined in the specification, processes for their preparation, pharmaceutical compositions containing them and their use in therapy. is less than about 35% in any direction, and such that the filler cloth maintains flame and heat resistant integrity when impinged with a gas flame in accordance with testing protocols set forth in Technical Bulletin 603 of the State of California Department of Consumer Affairs. In addition, or alternatively, other factors regarding the image may be utilized to determine whether or not caching of the image should occur. A discrete Fourier transform process and a phase compensation process in a frequency domain are performed. The amplifier is mounted in the container of the body and has at least one woofer mounted in the container adjacent to the through holes and two speaker mounted in the container.