def convert_pattern_to_string(pattern):
    """
    Converts a pattern to explicit format where "-" indicates the beginning of an
    itemset inside the main pattern and "--" indicates that the next item belongs to that itemset.
    Each "-" means the begin of a new itemset of length greater than 1.

    Atributes:
        original_pattern: Pattern to convert

    Example:
        [C1,C2,[C3,C1],[C2,C3,C4]] => [C1,C2,-C3,--C1,-C2,--C3,--C4]

    """
    result_pattern = []
    for ix in pattern:
        # Flag to check if it's the beginning of an itemset inside pattern.
        first = True
        # Check if the item is an itemset of length greater than 1 (list).
        if isinstance(ix, list):
            for jj in ix:
                if first:
                    # If it's the beginning, add a "-"
                    result_pattern.append('-'+jj)
                    first = False
                else:
                    # If it's the continuation of the itemset, add "--"
                    result_pattern.append('--'+jj)
        else:
            result_pattern.append(ix)

    return result_pattern