Hi, I have encountered this checkbounds function which confused me. Could anyone explain a bit what it does?

```
a_star_algorithm(g::AbstractGraph{U},
s::Integer,
t::Integer,
distmx::AbstractMatrix{T}=LightGraphs.weights(g),
heuristic::Function = (u,v) -> zero(T)) where {T, U}
**Arguments**
* `g` : graph object
* `S` : start vertex
* `t` : end vertex
* `distmx` : distance matrix
* `heuristic` : search heuristic function; by default returns zero
"""
function a_star_algorithm(g::LightGraphs.AbstractGraph{U}, # the g
s::Integer, # the start vertex
t::Integer, # the end vertex
distmx::AbstractMatrix{T}=LightGraphs.weights(g),
heuristic::Function = (u,v) -> zero(T)) where {T, U}
checkbounds(distmx, Base.OneTo(nv(g)), Base.OneTo(nv(g)))
#.....
```