diff --git a/command.py b/command.py
index 2e8f3e6..10ebdc9 100644
--- a/command.py
+++ b/command.py
@@ -1,22 +1,20 @@
 from panflute import Div,Span
 from util import *
-from typing import Callable
+from typing import List
 
 class Command:
 	pass
 
 class InlineCommand(Span, Command):
-	def call(self, f: Callable) -> Span:
-		r = f(self)
+	def replaceSelf(self, content: List[Element]) -> Span:
 		if "c" in r.attributes:
 			del r.attributes["c"]
-		return replaceEl(self, Span(*r.content, identifier=r.identifier, classes=r.classes, attributes=r.attributes))
+		return replaceEl(self, Span(*content))
 	pass
 
 class MultilineCommand(Div, Command):
-	def call(self, f: Callable) -> Div:
-		r = f(self)
+	def replaceSelf(self, content: List[Element]) -> Div:
 		if "c" in r.attributes:
 			del r.attributes["c"]
-		return replaceEl(self, Div(*r.content, identifier=r.identifier, classes=r.classes, attributes=r.attributes))
+		return replaceEl(self, Div(*content))
 	pass
diff --git a/formatitko.py b/formatitko.py
old mode 100644
new mode 100755
index 9c15dbe..60c6940
--- a/formatitko.py
+++ b/formatitko.py
@@ -9,12 +9,44 @@ import builtin_commands
 from panflute import *
 import re
 import sys
+from typing import List
 
 from mj_show import show
 
 flags = ["dog"]
+commands = []
 
-def transform(e: Element):
+def executeCommand(source: Code, element: Element) -> List[Element]:
+	mode = 'empty'
+	text = ""
+	content = []
+	def print(s: str):
+		if mode == 'elements':
+			raise SyntaxError("Cannot use `print` and `appendChild` in one command at the same time.")
+		mode = 'text'
+		text += s
+	
+	def println(s: str):
+		print(s+"\n")
+
+	def appendChild(e: Element):
+		if mode == 'text':
+			raise SyntaxError("Cannot use `print` and `appendChild` in one command at the same time.")
+		mode = 'elements'
+		content.append(e)
+
+	exec(source)
+
+	if mode == 'text':
+		return convert_text(text)
+	if mode == 'elements':
+		return content
+
+	
+
+
+
+def transform(e: Element) -> Element: # Returns next sibling element to transform
 	"""Transform the AST, making format-agnostic changes."""
 	
 	if isinstance(e, Whitespace) and bavlna(e):
@@ -40,49 +72,53 @@ def transform(e: Element):
 				e = replaceEl(e, InlineCommand(*e.content, identifier=e.identifier, classes=e.classes, attributes=e.attributes))
 	
 	
+	# Execute python code inside source code block
 	if isinstance(e, CodeBlock) and hasattr(e, "classes") and "python" in e.classes and "run" in e.classes:
 		exec(e.text)
 		deleteEl(e)
 		return
 	
+	
+	if isinstance(e, CodeBlock) and hasattr(e, "classes") and "python" in e.classes and hasattr(e, attributes):
+		if "define" in e.attributes:
+			if not e.attributes["define"] in commands:
+				commands[e.attributes["define"]] = compile(e.text, '<string>', 'exec')
+			else:
+				raise NameError(f"Command already defined: '{e.attributes['define']}'")
+		if "redefine" in e.attributes:
+			commands[e.attributes["redefine"]] = compile(e.text, '<string>', 'exec')
+	
 	# Handle special command shorthand [!commandname]{}
 	if isinstance(e, Span) and len(e.content) == 1 and isinstance(e.content[0], Str) and re.match(r"^![\w.]+$", e.content[0].text):
 		e = replaceEl(e, InlineCommand(identifier=e.identifier, classes=e.classes, attributes={**e.attributes, "c": e.content[0].text[1:]}))
 
 	# The command is executed first and then its contents are transformed. TODO: Is this what we want?
 	if isinstance(e, Command):
-		if re.match(r"^[\w.]+$", e.attributes["c"]):
-			try:
-				e = e.call(eval(e.attributes["c"]))
-			except NameError:
-				raise NameError(f"Command not found: '{e.attributes['c']}'.")
-		else:
-			raise SyntaxError(f"Invalid command syntax: '{e.attributes['c']}'.")
-	
-	# Recursively transform all contents.
-	if hasattr(e, "content"):
-		for c in e.content:
-			# Deleted elements are only marked to be deleted with a class, because otherwise
-			# it would mess up this for cycle.
-			transform(c)
-
-		# Delete elements marked to be deleted. This is somewhat inelegant.
-		ci = 0
-		while ci < len(e.content):
-			c = e.content[ci]
-			if hasattr(c, "classes") and "deleted" in c.classes:
-				del e.content[ci]
-			else:
-				ci+=1
+		if not e.attributes["c"] in commands:
+			raise NameError(f"Command not defined '{e.attributes['c']}'.")
+		e = e.replaceSelf(executeCommand(commands[e.attributes["c"]], e))
+
+	# Transformation of other elements. This is not done using a for cycle,
+	# because deletions of elements would mess with it.
+	children = ((child_name, getattr(e, child_name)) for child_name in e._children)
+	for child_name, child in children:
+		if hasattr(e, "content") and len(e.content) > 0:
+			next = transform(e.content[0])
+			while next != None:
+				next = transform(next)
+				print(next)
 
+	# Return nearest sibling to transform
+	return e.next
 
 
 
 doc = convert_text(open(sys.argv[1], "r").read(), standalone=True, extra_args=["-f", "markdown+fenced_code_attributes"])
 
 print(show(doc))
-transform(doc)
-print("---------------------")
-print(convert_text(doc, input_format="panflute", output_format="markdown"))
+print(doc.content._children)
+#transform(doc)
+#print("---------------------")
+#print(convert_text(doc, input_format="panflute", output_format="markdown"))
 
 
diff --git a/util.py b/util.py
index cfade95..d70cc0d 100644
--- a/util.py
+++ b/util.py
@@ -7,4 +7,4 @@ def replaceEl(e: Element, r: Element) -> Element:
 	r.parent = parent
 	return r
 def deleteEl(e: Element):
-	e.classes.append("deleted")
+	del e.parent.content[e.index]