text: between surface and depth information. The surface information of a sentence is the information it 
  presents explicitly. Depth information is the totality of the information contained in the sentence, both 
  explicitly and implicitly. According to this view, deductive inferences are uninformative on the depth 
  level. But they can be highly informative on the surface level by making implicit information explicit. 
  This happens, for example, in mathematical proofs.[73] 