Examples of expressions:
<blockquote><code>
-x
-(y x)
-(x x)
-(\x y)
-(\x x)
-(\x (\y x))
-(x (\x x))
-((\x (x x)) (\x (x x)))
+x<p>
+(y x)<p>
+(x x)<p>
+(\x y)<p>
+(\x x)<p>
+(\x (\y x))<p>
+(x (\x x))<p>
+((\x (x x)) (\x (x x)))<p>
</code></blockquote>
The lambda calculus has an associated proof theory. For now, we can regard the