WP_Block_Parser::proceed

	function proceed() {
		$next_token = $this->next_token();
		list( $token_type, $block_name, $attrs, $start_offset, $token_length ) = $next_token;
		$stack_depth = count( $this->stack );

		// we may have some HTML soup before the next block.
		$leading_html_start = $start_offset > $this->offset ? $this->offset : null;

		switch ( $token_type ) {
			case 'no-more-tokens':
				// if not in a block then flush output.
				if ( 0 === $stack_depth ) {
					$this->add_freeform();
					return false;
				}

				/*
				 * Otherwise we have a problem
				 * This is an error
				 *
				 * we have options
				 * - treat it all as freeform text
				 * - assume an implicit closer (easiest when not nesting)
				 */

				// for the easy case we'll assume an implicit closer.
				if ( 1 === $stack_depth ) {
					$this->add_block_from_stack();
					return false;
				}

				/*
				 * for the nested case where it's more difficult we'll
				 * have to assume that multiple closers are missing
				 * and so we'll collapse the whole stack piecewise
				 */
				while ( 0 < count( $this->stack ) ) {
					$this->add_block_from_stack();
				}
				return false;

			case 'void-block':
				/*
				 * easy case is if we stumbled upon a void block
				 * in the top-level of the document
				 */
				if ( 0 === $stack_depth ) {
					if ( isset( $leading_html_start ) ) {
						$this->output[] = (array) self::freeform(
							substr(
								$this->document,
								$leading_html_start,
								$start_offset - $leading_html_start
							)
						);
					}

					$this->output[] = (array) new WP_Block_Parser_Block( $block_name, $attrs, array(), '', array() );
					$this->offset   = $start_offset + $token_length;
					return true;
				}

				// otherwise we found an inner block.
				$this->add_inner_block(
					new WP_Block_Parser_Block( $block_name, $attrs, array(), '', array() ),
					$start_offset,
					$token_length
				);
				$this->offset = $start_offset + $token_length;
				return true;

			case 'block-opener':
				// track all newly-opened blocks on the stack.
				array_push(
					$this->stack,
					new WP_Block_Parser_Frame(
						new WP_Block_Parser_Block( $block_name, $attrs, array(), '', array() ),
						$start_offset,
						$token_length,
						$start_offset + $token_length,
						$leading_html_start
					)
				);
				$this->offset = $start_offset + $token_length;
				return true;

			case 'block-closer':
				/*
				 * if we're missing an opener we're in trouble
				 * This is an error
				 */
				if ( 0 === $stack_depth ) {
					/*
					 * we have options
					 * - assume an implicit opener
					 * - assume _this_ is the opener
					 * - give up and close out the document
					 */
					$this->add_freeform();
					return false;
				}

				// if we're not nesting then this is easy - close the block.
				if ( 1 === $stack_depth ) {
					$this->add_block_from_stack( $start_offset );
					$this->offset = $start_offset + $token_length;
					return true;
				}

				/*
				 * otherwise we're nested and we have to close out the current
				 * block and add it as a new innerBlock to the parent
				 */
				$stack_top                        = array_pop( $this->stack );
				$html                             = substr( $this->document, $stack_top->prev_offset, $start_offset - $stack_top->prev_offset );
				$stack_top->block->innerHTML     .= $html;
				$stack_top->block->innerContent[] = $html;
				$stack_top->prev_offset           = $start_offset + $token_length;

				$this->add_inner_block(
					$stack_top->block,
					$stack_top->token_start,
					$stack_top->token_length,
					$start_offset + $token_length
				);
				$this->offset = $start_offset + $token_length;
				return true;

			default:
				// This is an error.
				$this->add_freeform();
				return false;
		}
	}