4-ary mekso operator: Taylor expansion/polynomial term; for ordered input (X1, X, output is the X3th Taylor polynomial term of at-least-X3-smooth function X2 which was expanded around point X4 and which is evaluated at point X1, namely (1/(X3!)) × (D^X.
All of the usual assumptions must apply in order to be well-defined. X3 must be a nonnegative integer. X2 must be a function with at least X3 derivatives on the interval disc/interval defined by X1 and X4 such that said derivative takes values for which the other operators make sense (and finity is usually assumed). X1 and X4 must be elements of the domain of X2 (and the X3th derivative thereof in the latter case). The notation "(D^X3(X" represents the X3th derivative of X2, applied to X4; the "!" notation represents the factorial.