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conditionnal rule


hi all !!

I want to apply a rule under a certain condition. here is the example : 

myList = {{a, b}, {c, d}};

myList //. {a,x_}->{a,d}

apply this rule if Length[x]===1 (this means that x can not be a list)

I tried something like : 

myList //. {a_,x_}->{c,d}/;Length[x]===1

or

myList //. {a_,x_?Length[x]===1}->{c,d}/

but it does not work.

can someone help me ?


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