  We define a new cost model for the call-by-value lambda-calculus satisfying
the invariance thesis. That is, under the proposed cost model, Turing machines
and the call-by-value lambda-calculus can simulate each other within a
polynomial time overhead. The model only relies on combinatorial properties of
usual beta-reduction, without any reference to a specific machine or evaluator.
In particular, the cost of a single beta reduction is proportional to the
difference between the size of the redex and the size of the reduct. In this
way, the total cost of normalizing a lambda term will take into account the
size of all intermediate results (as well as the number of steps to normal
form).
