{a:b, c:d} // FullForm

set=Table[i->i^2, {i,10}]

DeleteCases[set, 4->_]

∖ is typset as esc-\-esc.

a∖b

a_∖key_ := DeleteCases[a, key -> _];
a_ ∖ keys_List := Fold[#1∖#2&, a, keys];

(set∖7)∖6

set∖{2,5}

?MapAt

MapAt[f, set, {All,2}]

(Subscript[((SuperPlus[R])), df]) // FullForm

(Subscript[vf, mm]^kk) // FullForm

(ragag)

egft[[rwag]]

R /: ((Subscript[R, a,b])^daf):=9

(Subscript[((SuperPlus[R])), df]):=9

(SuperPlus[R])[a,b]=9

{1,2,3}⋃{3,4,5}

StringLength["υ"]

StringLength["(OverBar[υ])"]