better explanation following Oliver's suggestions.

theano/scan_module/scan_utils.py

@@ -274,14 +274,14 @@ def equal_computations(xs, ys, in_xs=None, in_ys=None):
     The two lists `xs`, `ys` should have the same number of entries. The
     function checks if for any corresponding pair `(x,y)` from `zip(xs,ys)`
     `x` and `y` represent the same computations on the same variables
-    (unless equivalences are provided unsing `in_xs`, `in_ys`)
-
-    If `in_xs` and `in_ys` are provided, then these nodes are considered
-    equivalent even if they do not compare equal (they should however have
-    the same type). These lists could be used for example to provide
-    equivalence between inputs of two different graphs if what we want is
-    actually to see if (regardless of the input units) the two graph
-    actually perform the same computations on them.
+    (unless equivalences are provided unsing `in_xs`, `in_ys`).
+
+    If `in_xs` and `in_ys` are provided, then when comparing a node `x` with
+    a node `y` they are automatically considered as equal if there is some
+    index `i` such that `x == in_xs[i]` and `y == in_ys[i]`(and they both
+    have the same type). Note that `x` and `y` can be in the list `xs` and
+    `ys`, but also represent sub subgraph of a computational graph in `xs`
+    or `ys`.
 
     '''
     if in_xs is None: