Cases2[expr, f] returns a list of all objects in expr with head f.
Cases2[expr,f] is equivalent to Cases2[{expr},f[___],Infinity]//Union.
Cases2[expr, f, g, ...] or Cases2[expr, {f,g, ...}] is equivalent to Cases[{expr},f[___] | g[___] ...].
Cases2[f[a] + f[b]^2 + f[c, d], f]\{f(a),f(b),f(c,d)\}
Cases2[Sin[x] Sin[y - z] + g[y], Sin, g]\{g(y),\sin (x),\sin (y-z)\}
Cases2[Sin[x] Sin[y - z] + g[x] + g[a, b, c], {Sin, g}]\{g(x),g(a,b,c),\sin (x),\sin (y-z)\}
Cases2[GS[p] . GS[q] + SP[p, p], Dot]\left\{\left(\bar{\gamma }\cdot \overline{p}\right).\left(\bar{\gamma }\cdot \overline{q}\right)\right\}