Finds the layer used on the specified scheme.tl file.

def find_layer(file_path):
    """Finds the layer used on the specified scheme.tl file."""
    layer_regex = re.compile(r'^//\s*LAYER\s*(\d+)$')
    with file_path.open('r') as file:
        for line in file:
            match = layer_regex.match(line)
            if match:
                return int(match.group(1))