/* Contract indices in expression */

/* NOTE: buildq works like splicing a list with quasiquote in lisp; escaped
items are defined at start of quote */
contr(expr, var) ::= (
  if listp(var) and length(var) > 1 then
  block([head, tail],
    head : first(var),
    tail : rest(var),
    buildq([expr, head, tail], sum(contr(expr, tail), head, 1, 4)))
  elseif listp(var) and length(var) = 1 then
  block([head],
    head : first(var),
    buildq([expr, head], sum(expr, head, 1, 4)))
  elseif listp(var) then
  expr
  else
  buildq([expr, var], sum(expr, var, 1, 4)))$

/* Differentiate with respect to nth coordinate */
cdiff(func, coord) := diff(func, ct_coords[coord])$