Correction of description, claims and drawings 
Where the mistake is in the description, claims or drawings, both the error and the correction must be such that it is immediately evident (at least once attention is directed to the matter):
that an error has occurred; and 
what the correction should be. 
Regarding (i), the incorrect information must be objectively recognisable for a skilled person, using common general knowledge, from the originally-filed application documents (description, claims and drawings) taken by themselves. 
Regarding (ii), the correction should be within the limits of what a skilled person would derive directly and unambiguously, using common general knowledge, and seen objectively and relative to the date of filing, from the originally-filed application documents. 

In other words, the requirements of Art. 123(2) apply mutatis mutandis.

Evidence of what was common general knowledge on the date of filing may be furnished in any suitable form. 
The priority documents cannot be used for the purposes mentioned under (i) and (ii) above (see G 3/89 and G 11/91).

Correction under Rule 139, second sentence, is of a strictly declaratory nature and establishes what a skilled person, using common general knowledge, would already derive on the date of filing from the parts of a European patent application, seen as a whole, relating to the disclosure (see G 3/89 and G 11/91 mentioned above). Therefore, the complete replacement of the application documents (i.e. description, claims and drawings) by other documents is not possible (see G 2/95).

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