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用YACC实现的一个小的编译工具

EXAMPLE 2: SUBTYPES

This example program shows how values of domain types
may be used like objects.

The "->" notation can be used to access and update their fields.
Subtypes allow to manipulate the fields in type-safe way
outside of match statements.

The example uses the Tree type from Example 1 and gives functions to
update the fields of a node.

Note that the possibility to update fields introduces the full
power of general graph structures.