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diff --git a/src/jrw/Environment.java b/src/jrw/Environment.java
new file mode 100644
index 0000000..a93b149
--- /dev/null
+++ b/src/jrw/Environment.java
@@ -0,0 +1,85 @@
+package jrw;
+
+import java.util.*;
+import java.util.function.*;
+
+public class Environment {
+    public static final Environment root = new Environment();
+    private static final ThreadLocal<Environment> current = new ThreadLocal<>();
+    public final Environment parent;
+    private Map<Variable<?>, Object> data = Collections.emptyMap();
+
+    public static class Variable<T> {
+	private final Supplier<? extends T> ival;
+	private T rval;
+	private boolean inited;
+
+	public Variable(Supplier<? extends T> ival) {
+	    this.ival = ival;
+	}
+
+	@SuppressWarnings("unchecked")
+	public T get() {
+	    for(Environment env = current(); env != null; env = env.parent) {
+		Map<Variable<?>, Object> data = env.data;
+		if(data.containsKey(this))
+		    return((T)data.get(this));
+	    }
+	    if(!inited) {
+		synchronized(this) {
+		    if(!inited) {
+			rval = (this.ival == null) ? null : this.ival.get();
+			inited = true;
+		    }
+		}
+	    }
+	    return(rval);
+	}
+    }
+
+    public Environment(Environment parent) {
+	this.parent = parent;
+    }
+
+    public Environment() {
+	this(current());
+    }
+
+    public <T> void set(Variable<T> var, T val) {
+	synchronized(this) {
+	    Map<Variable<?>, Object> data = new IdentityHashMap<>(this.data);
+	    data.put(var, val);
+	    this.data = data;
+	}
+    }
+
+    public <T> void clear(Variable<T> var, T val) {
+	synchronized(this) {
+	    Map<Variable<?>, Object> data = new IdentityHashMap<>(this.data);
+	    data.remove(var);
+	    this.data = data;
+	}
+    }
+
+    public static Environment current() {
+	Environment ret = current.get();
+	return((ret == null) ? root : ret);
+    }
+
+    public class Frame implements AutoCloseable {
+	private final Environment prev;
+
+	private Frame() {
+	    this.prev = current.get();
+	    current.set(Environment.this);
+	}
+
+	public void close() {
+	    current.set(prev);
+	}
+    }
+
+    public Frame frame() {
+	return(new Frame());
+    }
+}