import re

VAR_RE = re.compile(r'\{\{([a-zA-Z_][a-zA-Z0-9_]*)\}\}')


def extract_vars(template: str) -> list[str]:
    seen = set()
    result = []
    for m in VAR_RE.finditer(template):
        if m.group(1) not in seen:
            seen.add(m.group(1))
            result.append(m.group(1))
    return result