  We present RSLR, an implicit higher-order characterization of the class PP of
those problems which can be decided in probabilistic polynomial time with error
probability smaller than 1/2. Analogously, a (less implicit) characterization
of the class BPP can be obtained. RSLR is an extension of Hofmann's SLR with a
probabilistic primitive, which enjoys basic properties such as subject
reduction and confluence. Polynomial time soundness of RSLR is obtained by
syntactical means, as opposed to the standard literature on SLR-derived
systems, which use semantics in an essential way.
