public function maintain_taxonomy_terms_during_meta_updates( $meta_id, $object_id, $meta_key, $meta_value ) {
		if ( '_edit_lock' === strtolower( $meta_key ) ) {
			return;
		}

		// These are the unprefixed meta keys that have overlapping taxonomies.
		$overlapping_keys = self::overlapping_meta_keys();
		// unprefix the meta key.
		$unprefixed = apply_filters( 'invp_unprefix_meta_key', $meta_key );

		// does $meta_key have a corresponding taxonomy?
		if ( ! in_array( $unprefixed, array_keys( $overlapping_keys ), true ) ) {
			// No.
			return;
		}

		$taxonomy = $overlapping_keys[ $unprefixed ];

		/**
		 * If we are in the Availability taxonomy, the end of this method
		 * appends terms instead of replacing so that a vehicle can be both
		 * For Sale and Wholesale. If the $meta_value is For Sale, Sold or Sale
		 * Pending, we need to remove the other of these terms.
		 */
		if ( 'availability' === strtolower( $unprefixed )
			&& ! empty( $meta_value )
			&& in_array( INVP::sluggify( $meta_value ), array( 'for-sale', 'sold', 'sale-pending' ), true )
		) {
			$remove_term_ids = get_terms(
				array(
					'taxonomy' => $taxonomy,
					'fields'   => 'ids',
					'slug'     => array_diff( array( 'for-sale', 'sold', 'sale-pending' ), array( INVP::sluggify( $meta_value ) ) ),
				)
			);
			$this->hooks_remove();
			wp_remove_object_terms( $object_id, $remove_term_ids, $taxonomy );
			$this->hooks_add();
		}

		// if $meta_value is empty or we have deleted a meta value, remove a term.
		global $action;
		if ( empty( $meta_value ) || 'delete-meta' === $action ) {
			// remove a term actually.
			$terms = array();
			if ( 'availability' === strtolower( $taxonomy ) ) {
				$terms      = wp_get_object_terms( $object_id, $taxonomy );
				$term_count = count( $terms );
				for ( $t = 0; $t < $term_count; $t++ ) {
					if ( $unprefixed === $terms[ $t ]->slug ) {
						/**
						 * Both $unprefixed and $terms[$t]->slug are 'wholesale'
						 * or $unprefixed and $taxonomy are both 'availability'
						 */
						// trash this one.
						unset( $terms[ $t ] );
						break;
					}
				}
			}

			$this->hooks_remove();
			wp_set_object_terms( $object_id, array_column( $terms, 'term_id' ), $taxonomy );
			$this->hooks_add();
			return;
		}

		/**
		 * Wholesale is a term in the Availability taxonomy rather than a real
		 * boolean as the meta field suggests & is registered.
		 */
		if ( 'wholesale' === strtolower( $unprefixed ) ) {
			$meta_value = 'Wholesale';
		}

		// is there already a term for this $meta_value in the taxonomy?
		$term = get_term_by( 'slug', INVP::sluggify( $meta_value ), $taxonomy );
		if ( ! $term ) {
			// it's not a slug, what about a name?
			$term = get_term_by( 'name', $meta_value, $taxonomy );
			if ( ! $term ) {
				// No, create a term.
				$term_id_array = wp_insert_term(
					$meta_value,
					$taxonomy,
					array(
						'description' => $meta_value,
						'slug'        => INVP::sluggify( $meta_value ),
					)
				);
				if ( ! is_wp_error( $term_id_array ) && ! empty( $term_id_array['term_id'] ) ) {
					$term = get_term( $term_id_array['term_id'], $taxonomy );
				}
			}
		}

		/**
		 * Assign the new term for this $object_id. The Availability taxonomy
		 * holds For Sale/Sold/Sale Pending and Wholesale, so append in that
		 * taxonomy.
		 */
		$this->hooks_remove();
		wp_set_object_terms( $object_id, $term->term_id, $taxonomy, ( 'availability' === strtolower( $taxonomy ) ) );
		$this->hooks_add();
	}