In the TDF "machine" the problem is not lack of structure at its "assembly" level, but rather too much of it; one loses the sense of a TDF program because of its deeply nested structure. Also the naming conventions of TDF are designed to make them tractable to machine manipulation, rather than human reading and writing. However, the approach is basically the same. PL_TDF provides shorthand notations for the commonly occuring control structures and operations while still allowing one to use the standard TDF constructors which, in turn, may have shorthand notations for their parameters. The naming is always done by identifiers where the sort of the name is determined by its declaration, or by context.
The TDF derived from PL_TDF is guaranteed to be SORT correct; however, there is no SHAPE checking, so one can still make illegal TDF.
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