doc"""
   buildif(exprs...)

`buildif` builds an if-elseif-else construct programmatically.

`buildif` returns an expression equivalent to an if-elseif-else
construct, where, if the length of exprs is even, then the odd-indexed
elements of exprs are condition expressions and the even-indexed
elements are code blocks for the corresponding branches. If the length
of exprs is odd, then the last element is the default code block. In
the even case, the default is `nothing`.

```julia
 if expr[1]
      expr[2]
 elseif expr[3]
      expr[4]
 else
      expr[5]
 end
```
"""

function buildif(exprs...)
    n = length(exprs)
    n == 2 && return Expr(:if, exprs..., nothing)
    n == 3 && return Expr(:if, exprs...)
    ex = Expr(:if, exprs[1], exprs[2], nothing)
    exargs = ex.args
    for i in  3:2:(n-3)
        newif = Expr(:if, exprs[i], exprs[i+1], nothing)
        exargs[3] = newif
        exargs = newif.args
    end
    if iseven(n)
        exargs[3] = Expr(:if, exprs[n-1], exprs[n], nothing)
    else
        exargs[3] = Expr(:if, exprs[n-2], exprs[n-1], exprs[n])
    end
    ex
end