Add height and length definitions

nugget/expressions.py

@@ -123,6 +123,12 @@ def get_focus(self):
     def get_children(self):
         return []
 
+    def height(self):
+        return None
+
+    def length(self):
+        return None
+
 
 class Binary(Expr):
     """Binary operation."""
@@ -274,6 +280,12 @@ def get_focus(self):
     def get_children(self):
         return [self.lhs, self.rhs]
 
+    def height(self):
+        return 1 + max(self.lhs.height(), self.rhs.height())
+
+    def length(self):
+        return 1 + self.lhs.length() + self.rhs.length()
+
 
 class Atom(Expr):
     """Variable."""
@@ -304,6 +316,12 @@ def __hash__(self):
     def is_atom(self):
         return True
 
+    def height(self):
+        return 0
+
+    def length(self):
+        return 1
+
 
 class Focus(Expr):
     """Focused expression."""
@@ -414,3 +432,9 @@ def get_focus(self):
     def get_children(self):
         return [self.expr]
 
+    def height(self):
+        return 1 + self.expr.height()
+
+    def length(self):
+        return 1 + self.expr.length()
+