def METHOD_NAME(content):
    """Extract Acton source code from Markdown document
    We just get the first source block and it should look like:
        Source:
        ```acton
        actor main....
        ```
    The starting "Source:" line is significant. The source block should
    immediately follow it.
    """
    look_for = '^Source:$'
    source = ""
    for line in content:
        if look_for == '^Source:$' and re.search(look_for, line):
            look_for = f"^```{SRCNAME}$"
            continue
        if look_for == f"^```{SRCNAME}$":
            if not re.search(look_for, line):
                raise ValueError(f"Didn't find source block, ```{SRCNAME} should follow the Source: line")
            look_for = '^```$'
            continue
        if look_for == "^```$":
            if re.search(look_for, line):
                return source
            source += line
    if look_for == '^Source:$':
        raise ValueError("Didn't find source block, missing 'Source:' line")