function markCurrentBranchAsGood() {
      const funcInfo = peek(funcInfoStack)
      const currentBranchID = peek(funcInfo.branchIDStack)
      if (funcInfo.branchInfoMap[currentBranchID]) {
        funcInfo.branchInfoMap[currentBranchID].good = true
      }
      // else unreachable code
    }
funcInfoStack is a stack representing the stack of currently executing
  functions
example:
  funcInfoStack = [ { branchIDStack: [ 's1_1' ],
      branchInfoMap:
       { s1_1:
          { good: false,
            loc: <loc> } } },
    { branchIDStack: ['s2_1', 's2_4'],
      branchInfoMap:
       { s2_1:
          { good: false,
            loc: <loc> },
         s2_2:
          { good: true,
            loc: <loc> },
         s2_4:
          { good: false,
            loc: <loc> } } } ]
@type {FuncInfo[]}