4.1. Definition of terms

• "undefined": means that installers can perform any action, including refusing to translate the program. It can produce code with any effect, meaningful or meaningless.

• "shall": when the phrase "P shall be done" (or similar phrases involving "shall") is used, every installer must perform P.

• "should": when the phrase "P should be done" (or similar phrase involving "should") is used, installers are advised to perform P, and producer writers may assume it will be done if possible. This usage generally relates to optimisations which are recommended.

• "will": when the phrase "P will be true" (or similar phrases involving "will") is used to describe the composition of a TDF construct, the installer may assume that P holds without having to check it. If, in fact, a producer has produced TDF for which P does not hold, the effect is undefined.

• "target-defined": means that behaviour will be defined, but that it varies from one target machine to another. Each target installer shall define everything which is said to be "target-defined".

Installers are not expected to check that the TDF they are processing is well-formed, nor that undefined constructs are absent. If the TDF is not well-formed any effect is permitted.

Installers shall only implement optimisations which are correct in all circumstances. This correctness can only be shown by demonstrating the equivalence of the transformed program, from equivalences deducible from this specification or from the ordinary laws of arithmetic. No statements are made in this specification of the form "such and such an optimisation is permitted".

Fortran90 has a notion of mathematical equivalence which is not the same as TDF equivalence. It can be applied to transform programs provided parentheses in the text are not crossed. TDF does not acknowledge this concept. Such transformations would have to be applied in a context where the permitted changes are known.