shift/reduce conflicts

[÷ÌÁÄÉÍÉÒ òÙËÏ×]

                         Hello to everyone !

     I upgrade my MathExpression Converter step by step.

     After I  added  this  sequence (below) of  operators Bison told me about 
half-dozen shift/reduce conflicts.

     Still - the Converter works well.

     I'd be grateful if somebody will give his advice what does it mean.
     (I skipped actions in {} brackets to make the situation more clear).

                ======================================

                                 ...

expression :
               andorexpression
                        ;
andorexpression:
                 setexpression
               | andorexpression andor andorexpression  {
                                        }
     ;
setexpression :

               simpleexpression
             | simpleexpression setoper simpleexpression
                                        {
                                        }
        ;
simpleexpression:    
                      mathexpr
                  |   mathexpr relation mathexpr
                                            {
                                                }
        ;
mathexpr:
                        variable
                     |  mathexpr mathoper mathexpr
                                 {
                                 }
        ;
                                 ...
                ======================================

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P bI K O B  B. B.    MOCKBA