documentclass{article}
usepackage{amsmath}
usepackage{xparse}
DeclareDocumentCommandfff { O{1} O{A} O{2} } {#1 underset{l}{overset{#2}{to}} #3 }
begin{document}
[
fff quad fff[3][B][4]
]
end{document}

newcommand{parcial}[2]{frac{partial #1}{partial #2}}

parcial{f}{x}: