The convert(expr, `+`) calling sequence sums all the operands of expr.

The convert(expr, `*`) calling sequence multiplies all the operands of expr.

$\mathrm{convert}\left(\sin \left(x\right)\left(y-x\right)\,\mathrm{`+`}\right)$

$\sin \left(x\right)+y-x$

$\mathrm{convert}\left(\sin \left(x\right)+y-x\,\mathrm{`*`}\right)$

$-\sin \left(x\right)yx$

$\mathrm{convert}\left(f\left(a\,b\,c\right)\,\mathrm{`+`}\right)$

$a+b+c$