It would really nice to add some functions to band maths expression, the vital functions can be listed as follows:
mean (X), (scalar): Returns the average/mean value of matrix/band X
min (X), (scalar): Returns the minimum value of matrix/band X
max (X) (scalar): Returns the maximum value of matrix/band X
Std (X) (scalar): Returns the standard deviation value of matrix/band X
there is a lack of such a functions for implementation of some formula.