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diff --git a/goldfishterm/internal/emit.h b/goldfishterm/internal/emit.h
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+// Copyright 2021 Benjamin Barenblat
+//
+// 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
+//
+//     https://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.
+
+// An extremely simple EDSL for describing output to the terminal. It supports
+// exactly two instructions: "send some bytes to the terminal" and "sleep". The
+// string capability interpreter produces terms in this EDSL, which makes it
+// easier to test.
+
+#ifndef EC_GOLDFISHTERM_INTERNAL_EMIT_H_
+#define EC_GOLDFISHTERM_INTERNAL_EMIT_H_
+
+#include <stdint.h>
+
+#include <memory>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#include "third_party/abseil/absl/strings/str_cat.h"
+#include "third_party/abseil/absl/strings/string_view.h"
+#include "third_party/abseil/absl/time/time.h"
+#include "third_party/abseil/absl/types/span.h"
+
+namespace goldfishterm_internal {
+
+class EmitTerm {
+ public:
+  virtual void Emit(std::ostream&) const = 0;
+
+  virtual ~EmitTerm() noexcept = default;
+};
+
+class EmitBytes final : public EmitTerm {
+ public:
+  explicit EmitBytes() noexcept = default;
+
+  explicit EmitBytes(absl::string_view bytes) noexcept : bytes_(bytes) {}
+
+  EmitBytes(const EmitBytes&) = default;
+  EmitBytes& operator=(const EmitBytes&) = default;
+  EmitBytes(EmitBytes&&) noexcept = default;
+  EmitBytes& operator=(EmitBytes&&) noexcept = default;
+
+  bool empty() const noexcept { return bytes_.empty(); }
+
+  void Append(absl::string_view more) noexcept {
+    absl::StrAppend(&bytes_, more);
+  }
+
+  void Emit(std::ostream&) const override;
+
+  bool operator==(const EmitBytes& other) const noexcept {
+    return bytes_ == other.bytes_;
+  }
+
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const EmitBytes& emit) noexcept {
+    return out << "EmitBytes(" << emit.bytes_ << ')';
+  }
+
+ private:
+  std::string bytes_;
+};
+
+class EmitDelay final : public EmitTerm {
+ public:
+  explicit EmitDelay() noexcept = default;
+
+  explicit EmitDelay(absl::Duration delay) : delay_(std::move(delay)) {}
+
+  EmitDelay(const EmitDelay&) = default;
+  EmitDelay& operator=(const EmitDelay&) = default;
+  EmitDelay(EmitDelay&&) noexcept = default;
+  EmitDelay& operator=(EmitDelay&&) noexcept = default;
+
+  bool zero() const noexcept { return delay_ == absl::ZeroDuration(); }
+
+  void Increase(absl::Duration more) noexcept { delay_ += more; }
+
+  void Emit(std::ostream&) const noexcept override;
+
+  bool operator==(const EmitDelay& other) const noexcept {
+    return delay_ == other.delay_;
+  }
+
+ private:
+  absl::Duration delay_;
+};
+
+// Runs an entire program in the emitter EDSL.
+void Emit(absl::Span<const std::shared_ptr<const EmitTerm>>, std::ostream&);
+
+}  // namespace goldfishterm_internal
+
+#endif  // EC_GOLDFISHTERM_INTERNAL_EMIT_H_