Labels multiply defined

[fingolfin]

Building the docs yields this:

#W There were LaTeX Warnings:
LaTeX Warning: Label `ElementsOfIncidenceStructure' multiply defined.


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LaTeX Warning: Label `ElementsOfIncidenceStructure' multiply defined.


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LaTeX Warning: Label `NrElementsOfIncidenceStructure' multiply defined.


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LaTeX Warning: Label `Rank' multiply defined.

I've disabled "warnings as errors" for the manuals in f8cc82c

But this should ideally be fixed. For example:

$ git grep -n '"ElementsOfIncidenceStructure"' doc
doc/affine.xml:161:<Oper Name="ElementsOfIncidenceStructure" Arg="as, j"/>
doc/classicalpolarspaces.xml:568:<Oper Name="ElementsOfIncidenceStructure" Arg="ps, j"/>
doc/incidence_geometry.xml:269:<Oper Name="ElementsOfIncidenceStructure" Arg="inc, j"/>
doc/incidence_geometry.xml:270:<Oper Name="ElementsOfIncidenceStructure" Arg="inc, str"/>
doc/incidence_geometry.xml:290:<Oper Name="ElementsOfIncidenceStructure" Arg="inc"/>
doc/projectivespaces.xml:171:<Oper Name="ElementsOfIncidenceStructure" Arg="ps, j"/>

All (or all but one) of these should have a Label="..." attribute so that GAPDoc can differentiate them in the index.