# R, <s>78</s> 71 bytes 

-7 bytes from aliasing the two longer function names and now taking user input from `scan()`

 a=scan(,'');s=strsplit;u=unlist;b=s(a[1],'');for(i in 2:length(a))b[[i]]=vecsets::vsetdiff(u(s(a[i],'')),u(b));b

This is my very first PPCG submission of any kind! So I have no idea what I'm doing both in terms of golfing and in terms of posting etiquette. The output is a list of vectors which may or may not meet the terms of the challenge. :-P

As for the code itself, it takes user input via `scan()` and compares each new day's letters to the cumulatively owned letters, as in other solutions. If there are shorter alternatives to `unlist` and `strsplit` for converting strings into vectors of individual characters that would be cool to know. I also used the `vsetdiff` function in Carl Withoft's `vecsets` package to get the set difference of the letters needed for the next day and the current letters owned.