[datatable-help] What's the trick to paste several columns together in one

[statquant3]

R) data.table(x=letters[1:5],y=rnorm(5),z=LETTERS[1:5])
    x                     y  z
1: a  0.5551301186 A
2: b  0.1092535710 B
3: c -2.4319115661 C
4: d  1.6915588561 D
5: e -1.5163715976 E

colNames = c('y','z')
newName = 't'

How do I add a column 't' such that t:=paste(y,x,sep='_')

    x                     y  z t
1: a  0.5551301186 A 0.5551301186_A
2: b  0.1092535710 B 0.1092535710_B
3: c -2.4319115661 C ...
4: d  1.6915588561 D ...
5: e -1.5163715976 E ...




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