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It has become a standard approach to reason about contextual equivalence using some notion of (bi)simulation. The main technical task of this approach is to show that (bi)simulation is a (pre)congruence, and hence is sound for reasoning about contextual equivalence. Howe devised a method to prove this, and his method has been widely adapted and applied. This paper deals with this challenge for call-by-need computation including non-determinism. In this setting, sharing of sub-computations mustdoi:10.1016/j.ic.2009.11.003 fatcat:32rimryxd5evll6xbskmnvj4wa