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+## std/iterators: The Iter interface and associated functions.
+## This module is imported by default.
+
+## The Iter interface. Any type implementing `next()` is iterable.
+pub type Iter[T] = interface
+  next(mut Self): Option[T]
+
+# todo: useful functions for an iterator
+# https://doc.rust-lang.org/std/iter/trait.Iterator.html#provided-methods
+
+pub func advance_by[T](self: Iter[T], n: uint): Result[T, ...] =
+  for i in 0 .. n:
+    if self.next().is_none():
+      return Error(...)
+  Okay
+
+pub func get[T](self: Iter[T], at: uint): Option[T]
+  self.advance_by(at-1).ok?
+  self.next()
+
+# todo: implement iter[T](self: ...): Iter[T] funcs
+# todo: efficient functional methods
+
+## The Peek interface. Any type implementing Iter, `peek`, and `peek_nth` is peekable.
+pub type Peek[T] = interface
+  next(mut Self): Option[T]
+  peek(mut Self): Option[T]
+  peek_nth(mut Self, n: uint): Option[T]
+
+# todo: implement peek[T](self: Iter[T]): Peek[T]
+# todo: implement Peekable struct
+# https://github.com/LukeMathWalker/multipeek/blob/main/src/lib.rs
+
+## We don't want a Countable. It's not terribly useful.
+# pub type Countable[T] = interface
+#   next(mut Self): Option[T]
+#   len(Self): uint
+#   get(Self, uint): Option[T]