4.3.1 - if then else

An if statement tests its condition and executes its then-part or its else-part accordingly. The else-part is optional.

    if a<0 then a = 0 end
    
    if a<b then return a else return b end
    
    if line > MAXLINES then
      showpage()
      line = 0
    end
When you write nested ifs, you can use elseif. It is similar to an else followed by an if, but it avoids the need for multiple ends:
    if op == "+" then
      r = a + b
    elseif op == "-" then
      r = a - b
    elseif op == "*" then
      r = a*b
    elseif op == "/" then
      r = a/b
    else
      error("invalid operation")
    end