Operator acting on hmatrix preserving the attributes when rows are extracted.

```
# S3 method for hmatrix
[(x, i, j, ..., drop = FALSE)
```

x

object from which to extract element(s) or in which to replace element(s).

i, j

indices specifying elements to extract or replace. Indices are numeric vectors or empty (missing) or NULL. Numeric values are coerced to integer as by as.integer (and hence truncated towards zero).

...

not used

drop

If `TRUE`

the result is coerced to the lowest possible dimension
(or just a matrix). This only works for extracting elements, not for the
replacement, defaults to `FALSE`

.

If used on columns or rows/columns a matrix is returned.
If used on rows only, i.e. x[i,] an object of class hmatrix is returned.
The id is changed so that it runs from 1, ..., nNew, where nNew is the number of different
id values in the new hmatrix-object.
From the functional covariate `x`

rows are selected accordingly.

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