theory Base_FDS imports "HOL-Library.Pattern_Aliases" begin declare Let_def [simp] text ‹Lemma ‹size_prod_measure›, when declared with the ‹measure_function› attribute, enables ‹fun› to prove termination of a larger class of functions automatically. By default, ‹fun› only tries lexicographic combinations of the sizes of the parameters. With ‹size_prod_measure› enabled it also tries measures based on the sum of the sizes of different parameters. To alert the reader whenever such a more subtle termination proof is taking place the lemma is not enabled all the time but only when it is needed. › lemma size_prod_measure: "is_measure f ⟹ is_measure g ⟹ is_measure (size_prod f g)" by (rule is_measure_trivial) end