Modes, values and expressions

Several modern programming languages (e.g. Algol 68 [1], Pascal [2], Basel [3], EL1 [4] ) require each variable to have a declared mode or type, which limits the class of values wh~ch it may assume. In each of these languages, a value may contain a reference or pointer to z value of the same type, allow?ng l~nked lists. In this case, it is natural to define the mode of the value recursively -that is, in terms of itself. For example, if list describes values which are structures containing two fields, one being an integer and the other a pointer to another value of type list, then we can summarize these facts by~ting