Typo: Rename "estimetor.go" to "estimator.go".
Separate "(*Estimator).build" from "(*Estimator).Estimate".
Represent the parameters in a struct, which is named "Freq".
(*Estimator).build only requires a value typed as *Freq.
Separate "(*Estimator).count" from "(*Estimator).Estimate".
(*Freq).Add counts the frequencies of N-grams and their contexts.
Finish the first step of Refactoring "(*Estimator).Estimate".
Merge branch 'feature/estimator'