Returns a single value from a column from a different dataset, matching by the key.

def cross(environment, book, row, sheet_source, column_source, column_key):
    """
    Returns a single value from a column from a different dataset, matching by the key.
    """
    a = book.sheets[sheet_source]

    return environment.copy(a.get(**{column_key: row[column_key]})[column_source])