\newcommand{\abc}{5}
\newcommand{\xyz}[2][x]{%
#1 + #2%
}
\renewcommand{%
\phi%
}{%
\theta%
}
% comment
\newcommand{\aaa}{\b{2}}
\renewcommand{\phi}{\aaa}
\newcommand{\b}[1]{#1}

\phi
\abc
\xyz[y] {3}
\xyz 3
\xyz \alpha
