code: DateTimeBuilder addFieldValue(TemporalField field, long value) {
          Jdk8Methods.requireNonNull(field, "field");
          Long old = getFieldValue0(field);  // check first for better error message
          if (old != null && old.longValue() != value) {
              throw new DateTimeException("Conflict found: " + field + " " + old + " differs from " + field + " " + value + ": " + this);
          }
          return putFieldValue0(field, value);
      }
docs: Adds a field-value pair to the builder.
  <p>
  This adds a field to the builder.
  If the field is not already present, then the field-value pair is added to the map.
  If the field is already present and it has the same value as that specified, no action occurs.
  If the field is already present and it has a different value to that specified, then
  an exception is thrown.
  
  @param field  the field to add, not null
  @param value  the value to add, not null
  @return {@code this}, for method chaining
  @throws DateTimeException if the field is already present with a different value