Rで既定で有効になっているbaseパッケージには、cという名前の関数があります。これは連結(concatenate)ないし結合(combine)のための関数です。また、tという名前の関数があります。これは転置(transpose)関数です。
このように短い名前の識別子がもともと定義されている場合でも、変数名として別の値を束縛することは可能ですが、紛らわしいコードになる恐れがあります。そこで、気付かずに変数名に使ってしまわないように、3文字以下のアルファベットからなる既定で定義済みの識別子を以下のように一覧してみました。
% R
R version 3.6.3 (2020-02-29) -- "Holding the Windsock"
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Platform: x86_64-apple-darwin19.3.0 (64-bit)
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> Map(function(p) Filter(function(x) grepl('^\\w{1,3}$', x), ls(sprintf('package:%s', p))), c('base', getOption('defaultPackages')))
$base
[1] "abs" "all" "any" "Arg" "by" "c" "cat" "col" "cos" "cut" "det" "dim"
[13] "dir" "exp" "F" "for" "gc" "get" "gl" "I" "if" "Im" "log" "ls"
[25] "Map" "max" "min" "Mod" "pi" "q" "qr" "raw" "Re" "rep" "rev" "rle"
[37] "rm" "row" "seq" "sin" "sub" "sum" "svd" "t" "T" "tan" "try" "unz"
[49] "url" "xor"
$datasets
[1] "BOD" "co2" "CO2" "lh" "npk"
$utils
[1] "de" "fix" "nsl" "str" "tar" "vi" "zip"
$grDevices
[1] "bmp" "cm" "hcl" "hsv" "pdf" "png" "rgb" "svg" "x11" "X11"
$graphics
[1] "box" "bxp" "lcm" "par" "pie" "rug"
$stats
[1] "acf" "AIC" "aov" "ar" "ave" "BIC" "C" "ccf" "cor" "cov" "D" "df"
[13] "dt" "end" "fft" "glm" "hat" "IQR" "lag" "lm" "mad" "nlm" "nls" "pf"
[25] "ppr" "pt" "qf" "qt" "rf" "rt" "sd" "SSD" "stl" "ts" "tsp" "var"
$methods
[1] "as" "el" "is" "new" "Ops"
>
該当するものが100以上あることが分かります。また、小文字だけでなく、大文字のCとTも定義済みです。