[Agda] String -> Nat

[Serge D. Mechveliani]

Please,
what Standard library has for reading (parsing)  Nat  from String
(or List Char) ?
For example,  "10" -> suc suc suc suc suc suc suc suc suc suc zero.

I need to input Nat from a file, and use  readFile.
This produces  str : String  (may be Costring),  
then,  str  needs to convert to Nat.

Thanks,

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Sergei