code: public ConceptBuilder getBuilder(Variable var) {
          return tryBuilder(var).orElseThrow(() -> {
              Concept concept = concepts.get(equivalentVars.componentOf(var));
              return GraqlQueryException.insertExistingConcept(printableRepresentation(var), concept);
          });
      }
docs: Return a ConceptBuilder for given Variable. This can be used to provide information for how to create
  the concept that the variable represents.
  This method is expected to be called from implementations of
  VarProperty#insert(Variable), provided they return the given Variable in the
  response to PropertyExecutor#producedVars().
  For example, a property may call {@code executor.builder(var).isa(type);} in order to provide a type for a var.
  
  @throws GraqlQueryException if the concept in question has already been created