PullBack

 Author: Kurt Pagani 
 Date Created: Wed Jun 10 19:16:01 CEST 2020
 License: BSD
 References:
 Description:
  Given a mapping f:S->T, where S,T are presumed vector spaces,
  represented by the list of coordinate symbols S:=[u_1,..,u_m] and
  T:=[x_1,..,x_n], this package "PullBack(S,T)" provides functions
  that pull elements from DeRhamComplex(INT,T) back to elements
  of DeRhamComplex(INT,S) by the usual rules of exterior calculus.
  Instead of a mapping f, one can also provide a list of equations of
  the form [x_1=x_1(u),..,x_n=x_n(u)].

 pullBack(f,w) returns the pulled back form f*(w).

 pullBack([x=f(u)],w) returns the pulled back form f*(w).

 pullBack(f,g(x)) returns the expression g(f(u)).

 basisForms() returns a record containing the list of basis
 forms of DeRhamComplex(INT,S) and DeRhamComplex(INT,T).