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:first-child 
:not(:first-child)
:first-of-type
:last-child
/*Par*/
 &:nth-child(even) {
     margin-right: 0px !important;
  }
  
/*Ínpar*/
 &:nth-child(even) {
     margin-right: 0px !important;
 }
 
 /* a cada 3 itens */
 .gallery > div:nth-child(3n) {
    margin-right: 0px;
}

/*Mais de uma transition animation*/
  transition: max-height .5s ease-in-out, opacity .5s ease-in-out;
  -webkit-transition: max-height .5s ease-in-out, opacity .5s ease-in-out;
  -moz-transition: max-height .5s ease-in-out, opacity .5s ease-in-out;
  -o-transition: max-height .5s ease-in-out, opacity .5s ease-in-out;
  
  
  /*Selecionar seletor que nao contenha uma classe especifica*/
  li.course:not(.finished)