/* 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 buildq(
    [expr, head : first(var), tail : rest(var)],
    sum(contr(expr, tail), head, 1, 4))
  elseif listp(var) and length(var) = 1 then buildq(
    [expr, head : first(var)], 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])$

/* Wave operator */
box_op(func) := (
  expand(contr(ug[μ,ν] * cdiff(cdiff(func, ν), μ), [μ,ν])
    - contr(ug[μ,ν] * mcs[μ,ν,ρ] * cdiff(func, ρ), [μ,ν,ρ])))$