When I write `W[f_] := Integrate[f[x], {x, 0, π}]` I may rightly conclude that
`W[Sin] = 2`, `W[Cos] = 0`, `W[Log] = π (-1 + Log[π])` etc. But I am unable to define `W[Sin + Cos]` or `W[Sin[Sin]]` etc. Please help.