ok for example:

----------A(activities)--------B(predecessors)-------------C(successors)
1-----excavation--------------------------------------formwork, rebar
2------formwork---------------excavation--------------rebar, columns
3--------rebar-----------------formwork, excavation-----------columns
4-------columns------------rebar,formwork------------------

the contents of column A are manually entered.
For column B, cells have formulas like a1 & ", " & a2 & ", " & a3... as much terms from column A as needed, and they display the text there as you see.

The question is for column C. I am asking the formula to write in C. The text should display like I wrote as a result. What it must do is, for any row, say row2, it must scan column B, and, for each row in column B that mentioned the cell A1, the formula in C1 must include the corresponding cells from A.

So in this case, for row 2, C2 looks at column B. and sees that A2 was mentioned in B3 and B4. So, it writes A3 and A4 in its formula.

So in other words, whatever activities mentiones an activity as a predecessor, that predecessor activity must mention those activities as its successor automatically.