code: public Optional<ConceptBuilder> tryBuilder(Variable var) {
          var = equivalentVars.componentOf(var);
  
          if (concepts.containsKey(var)) {
              return Optional.empty();
          }
  
          ConceptBuilder builder = conceptBuilders.get(var);
  
          if (builder != null) {
              return Optional.of(builder);
          }
  
          builder = ConceptBuilder.of(this, var);
          conceptBuilders.put(var, builder);
          return Optional.of(builder);
      }
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 include the given Variable in
  their PropertyExecutor#producedVars().
  For example, a property may call {@code executor.builder(var).isa(type);} in order to provide a type for a var.
  If the concept has already been created, this will return empty.