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+// Copyright 2020 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package adt represents partially and fully evaluated CUE types.
+//
+// This package introduces several categories of types that indicate some set of
+// values that may be used in a certain situation. Concrete types may belong to
+// multiple categories.
+//
+//
+// Abstract Types
+//
+// The following types describe the a place where a value may be used:
+//
+//    Decl       a value than can be used as a StructLit element.
+//    Elem       a value than can be used as a ListLit element.
+//    Expr       represents an Expr in the CUE grammar.
+//    Value      a fully evaluated value that has no references (except for
+//               children in composite values).
+//    Node       any of the above values.
+//
+// The following types categorize nodes by function:
+//
+//    Resolver   a reference to position in the result tree.
+//    Evaluator  evaluates to 1 value.
+//    Yielder    evaluates to 0 or more values.
+//    Validator  validates another value.
+//
+//
+// Reference resolution algorithm
+//
+// A Resolver is resolved within the context of an Environment. In CUE, a
+// reference is evaluated by substituting it with a copy of the value to which
+// it refers. If the copied value itself contains references we can distinguish
+// two different cases. References that refer to values within the copied
+// reference (not regarding selectors) will henceforth point to the copied node.
+// References that point to outside the referened value will keep refering to
+// their original value.
+//
+//      a: b: {
+//        c: int
+//        d: c
+//        e: f
+//      }
+//      f: 4
+//      g: a.b { // d.c points to inside the referred value, e.f, not.
+//        c: 3
+//      }
+//
+// The implementation doesn't actually copy referred values, but rather resolves
+// references with the aid of an Environment. During compile time, each
+// references is associated with the label and a number indicating in which
+// parent scope (offset from the current) this label needs to be looked up. An
+// Environment keeps track of the point at which a value was referenced,
+// providing enough information to look up the labeled value. This Environment
+// is the identical for all references within a fields conjunct. Often, an
+// Environment can even be shared among conjuncts.
+//
+//
+// Values
+//
+// Values are fully evaluated expressions. As this means that all references
+// will have been eliminated, Values are fully defined without the need for an
+// Environment. Additionally, Values represent a fully evaluated form, stripped
+// of any comprehensions, optional fields or embeddings.
+//
+package adt