Derives a new variable according to a specified function of synthesised data.
syn.passive(data, func)a data frame with synthesised data.
a formula specifying transformations on data. 
    It is specified as a string starting with ~.
A vector including the result of applying the formula.
Any function of the synthesised data can be specified. Note that several operators such as 
  +, -, * and ^ have different meanings in formula syntax. Use 
  the identity function I() if they should be interpreted as arithmetic operators, 
  e.g. "~I(age^2)".
Van Buuren, S. and Groothuis-Oudshoorn, K. (2011).
  mice: Multivariate Imputation by Chained Equations
  in R. Journal of Statistical Software,
  45(3), 1-67.
  http://www.jstatsoft.org/v45/i03/