<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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])