1 | initial version |

If `v`

is in `R`

then you can use e.g. the condition `v[:3] == '$a_'`

to check that the first three characters in `v`

are `$a_`

.

```
sage: [v for v in R if v[:3] == '$a_']
['$a_{3}$', '$a_{5}$']
```

2 | No.2 Revision |

If `v`

is in `R`

then you can use e.g. the condition `v[:3] == '$a_'`

to check that the first three characters in `v`

are `$a_`

.

```
sage: [v for v in R if v[:3] == '$a_']
['$a_{3}$', '$a_{5}$']
sage: for v in R:
....: if v[:3] == '$a_':
....: print(v)
....:
$a_{3}$
$a_{5}$
```

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