\documentclass{article}

\usepackage{xargs}


\newcommand{\comm}[1]{Hello #1!}
\newcommandx{\commx}[3][2=xxX, 3=Xxx]{#2 Hello #1! #3}
\newcommandx*\coord[3][1=1, 3=n]{(#2\_{#1}, ..., #2\_{#3})}

\renewcommandx{\comm}[1]{Hello #1!!!}
\providecommandx{\comm}[5][2=24, 4=42]{foo bar baz}

\newenvironmentx{foo}{bar}{hum}
\renewenvironmentx{foo}[2][1=honey, 2=mustard]{``{#1} - ``{#2}''}{``The End''}

\newcommandx{\hihi}[2][1=honey, 2=mustard, usedefault]{``{#1}'' - ``{#2}''}
\newcommandx{\hihihi}[2][1=honey, 2=mustard, usedefault=ha]{``{#1}'' - ``{#2}''}

\newcommandx{\greeting}[2][1=Hello, 2=World]{#1 #2!}
\newcommandx{\greetingUsedef}[2][1=Hello, 2=World, usedefault]{#1 #2!}
\newcommandx{\greetingUsedefAt}[2][1=Hello, 2=World, usedefault=@]{#1 #2!}

\newcommandx{\prefixstuff}[1][addprefix=\global]{Prefix: #1}
\newcommandx*{\prefixstuffstar}[1][addprefix=\global]{** Prefix: #1 **}

\begin{document}

\comm{World} \\
\commx{World} \\

\coord{x} \\
\coord[0]{y} \\
\coord{z}[m] \\
\coord[0]{t}[m]

\begin{foo}
	butterfly
\end{foo}

\greeting \\
\greeting[Bye] \\
\greeting[][Tree] \\
\greetingUsedef[][Forrest] \\
\greetingUsedefAt[@][Sea] \\
\greetingUsedefAt[][Lake] \\

\prefixstuff{foo} \\
\prefixstuffstar{bar} \\

\end{document}