Narrowing the set of possible types

. Ada requires a complete expression to have a unique type

. Given a unique type from the context we can narrow down the type choices for each expression

. If this process does not result in a unique type for each sub expression then a type error is declared for the expression

Ada requires every expression to have one unique type. Hence when narrowing down the types in top down pass we make use of the fact that each expression has only one unique type. Refer to the next page for details.