The general scheme for writing commands will be something like this
(see `help language`, this is a pared down version):

````
[prefix:] command [varlist] [if] [, options]
````

For example, try:

```{stata, eval=FALSE}
help summarize
```

and you will see

````
summarize [varlist] [if] [in] [weight] [, options]
````

The name of the command is *summarize*.  It can be abbreviated as
short as *su* (although *sum* is more common in practice).  This is
indicated by the underlining.

This command takes ("accepts") a *variable list*.  The square brackets
indicate that the variable list is actually optional for this command,
as is everything besides the command name, *summarize*.

Having loaded a data set, we can try out this command

```{stata}
summarize
```

## *Varlist*

Next we add a variable list.  There are a number of ways we can 
write lists of variables in Stata.  (See `help varlist`.)

- write out the variable name(s)
- use wildcards
- use a first-to-last (sequential) list
- the keyword `_all`

```{stata}
summarize price mpg // spelled-out list
summarize mpg - trunk // first-to-last list
summarize *n   // wildcard list
summarize _all // or use " * "
```

Some languages have indexed lists, like *myvar1-myvar4*, meaning the
four variables myvar1, myvar2, myvar3, and myvar4, regardless of where
they are located in the data set.  Not Stata.  In Stata this would be
a first-to-last (positional) list.  It might be the list you want, or
it might not - it depends how the data set was constructed.

### Exercise
How else could you specify the list with myvar1, myvar2, myvar3, and 
myvar4?  (State any assumptions you make.)

## *If*
An `if` condition allows your command to act on a *subset* of the
observations in your data set.  (See `help if`.)  The syntax for
most commands can be

````
command if expression
````

where *expression* is something that is true or false for each observation
in the data set.

For example, to see the average price of a foreign car in this
data set we might try

```{stata}
summarize price if foreign==1
```