Statement
When the hammer does not work, use a sledge hammer.
Assumption
I. A sledge hammer is more powerful than a hammer.
II. Different tools are required in different conditions.

If A needs to be substituted by B, we assume that B is more appropriate than A. Now, the appropriateness of sledgehammer may be due to either of the following reasons: (a) a sledgehammer is more powerful; (b) though not more powerful, it works because different tools are required in different conditions. (b) may be explained by the following example : A chalk can not be said to be more powerful than a pen. But when it comes to writing on a blackboard, you would use a chalk, not a pen.