Can't only use optional arguments?

Example of what should be done:
```
./prog.lua build
```
and 
```
./prog.lua install
```
but if i make install and build both options i get the following error:
`mpm: bad number of arguments: 0 argument(s) must be specified, not 1; re-run with help for usage`
I can set build or install as arguments, but that makes both options REQUIRED, which is not what is wanted.
