Add docs for generic eval_tree_array

Author: MilesCranmer

src/EvaluateEquation.jl

@@ -432,6 +432,45 @@ function deg2_diff_eval(
     return (out, no_nans)
 end
 
+"""
+    eval_tree_array(tree::Node, cX::AbstractMatrix{T,N}, operators::GenericOperatorEnum) where {T,N}
+
+Evaluate a generic binary tree (equation) over a given input data,
+whatever that input data may be. The `operators` enum contains all
+of the operators used. Unlike `eval_tree_array` with the normal
+`OperatorEnum`, the array `cX` is sliced only along the first dimension.
+i.e., if `cX` is a vector, then the output of a feature node
+will be a scalar. If `cX` is a 3D tensor, then the output
+of a feature node will be a 2D tensor.
+Note also that `tree.feature` will index along the first axis of `cX`.
+
+However, there is no requirement about input and output types in general.
+You may set up your tree such that some operator nodes work on tensors, while
+other operator nodes work on scalars. `eval_tree_array` will simply
+return `nothing` if a given operator is not defined for the given input type.
+
+This function can be represented by the following pseudocode:
+
+```
+function eval(current_node)
+    if current_node is leaf
+        return current_node.value
+    elif current_node is degree 1
+        return current_node.operator(eval(current_node.left_child))
+    else
+        return current_node.operator(eval(current_node.left_child), eval(current_node.right_child))
+```
+
+
+
+# Returns
+
+- `(output, complete)::Tuple{Any, Bool}`: the result,
+    as well as if the evaluation completed successfully (true/false).
+    If evaluation failed, `nothing` will be returned for the first argument.
+    A `false` complete means an operator was called on input types
+    that it was not defined for.
+"""
 function eval_tree_array(
     tree, cX::AbstractArray{T,N}, operators::GenericOperatorEnum
 ) where {T,N}