How to initialise a seq in an object

[ElegantBeef]

Based off your error I believe the issue is that your object is 
immutable(declared with `let`). The following compiles fine.
    
    
    type
      ColInfo* = object
        `in`: string
        `out`: string
      
      TableInfo* = object
        cols*: seq[ColInfo]
    var ti = TableInfo()
    ti.cols.add ColInfo(`in`: "a", `out`: "b")
    
    
    Run