growth(price, signals, ...)
  rollFUN(x, n, FUN, ...)
  lags(x, n = 1)
  wilderSum(x, n = 10)growth returns a vector of the growth of the investment.
  rollFUN returns a vector of function results for the rolling subsets of data.
  lags returns a matrix of lagged values of the original vector.
  wilderSum returns a vector of weighted sums.growth calculates the growth of an investment using given prices and signals.
  rollFUN allows the calculation of rolling functions.
  lags calculates the lags of a given series.
  wilderSum calculates a Welles Wilder style weighted sum.